Suppression of mammary carcinoma growth in vitro and in vivo by inducible expression of the Cdk inhibitor p21.

Mammary carcinomas that develop in C3 (1)/SV40 T- antigen (TAg) transgenic mice have lost the p53-mediated induction of p21, leading to increased cellular proliferation and significant elevations of cyclins and Cdks. To test whether p21 could serve as a target for anticancer therapy for this mammary cancer model, a retroviral delivery system for the inducible expression of p21 was developed. We demonstrate that overexpression of p21 in C3(1)/TAg mammary tumor cells using the retroviral inducible p21 expression system results in increased apoptosis, reduced cell proliferation in vitro and reduced tumor growth in vivo associated with reduced expression of cyclins D1 and E, and Cdks 2, 4, and 6. Reciprocal changes in the expression of p21 and p27(Kip1), another cell-cycle regulator, were also observed. Because reduced p21 expression occurs frequently in human breast cancer, restoration of the Cdk inhibitor p21 by gene therapy approaches may provide a method for inhibiting mammary tumor progression.

Complete regression of established spontaneous mammary carcinoma and the therapeutic prevention of genetically programmed neoplastic transition by IL-12/pulse IL-2: induction of local T cell infiltration, Fas/Fas ligand gene expression, and mammary epithelial apoptosis.

Using a novel transgenic mouse model of spontaneous mammary carcinoma, we show here that the IL-12/pulse IL-2 combination can induce rapid and complete regression of well-established autochthonous tumor in a setting where the host immune system has been conditioned by the full dynamic process of neoplastic progression and tumorigenesis. Further, this regimen inhibits neovascularization of established mammary tumors, and does so in conjunction with potent local induction of genes encoding the IFN-gamma- and TNF-alpha-inducible antiangiogenic chemokines IFN-inducible protein 10 and monokine induced by IFN-gamma. In contrast to untreated juvenile C3(1)TAg mice in which histologically normal mammary epithelium predictably undergoes progressive hyperplasia, atypical changes, and ultimately transition to overt carcinoma, the current studies also demonstrate a unique preventative therapeutic role for IL-12/pulse IL-2. In juvenile mice, early administration of IL-12/pulse IL-2 markedly limits the expected genetically programmed neoplastic transition within the mammary epithelium and does so in conjunction with enhancement of constitutive Fas and pronounced induction of local Fas ligand gene expression, T cell infiltration, and induction of apoptosis within the mammary epithelium. These events occur in the absence of a durable Ag-specific memory response. Thus, this novel model system demonstrates that the potent therapeutic activity of the IL-12/pulse IL-2 combination rapidly engages potent apoptotic and antiangiogenic mechanisms that remain active during the delivery of IL-12/pulse IL-2. The results also demonstrate that these mechanisms are active against established tumor as well as developing preneoplastic lesions.

The C3(1)/SV40 T-antigen transgenic mouse model of mammary cancer: ductal epithelial cell targeting with multistage progression to carcinoma.

The 5' flanking region of the C3(1) component of the rat prostate steroid binding protein (PSBP) has been used to successfully target the expression of the SV40 large T-antigen (Tag) to the epithelium of both the mammary and prostate glands resulting in models of mammary and prostate cancers which histologically resemble the human diseases. Atypia of the mammary ductal epithelium develops at about 8 weeks of age, progressing to mammary intraepithelial neoplasia (resembling human ductal carcinoma in situ [DCIS]) at about 12 weeks of age with the development of invasive carcinomas at about 16 weeks of age in 100% of female mice. The carcinomas share features to what has been classified in human breast cancer as infiltrating ductal carcinomas. All FVB/N female mice carrying the transgene develop mammary cancer with about a 15% incidence of lung metastases. Approximately 10% of older male mice develop anaplastic mammary carcinomas. Unlike many other transgenic models in which hormones and pregnancy are used to induce a mammary phenotype, C3(1)/Tag mice develop mammary tumors in the mammary epithelium of virgin animals without hormone supplementation or pregnancy. Although mammary tumor development appears hormone-responsive at early stages, invasive carcinomas are hormone-independent, which corresponds to the loss of estrogen receptor-alpha expression during tumor progression. Molecular and biologic factors related to mammary tumor progression can be studied in this model since lesions evolve over a predictable time course. Genomic alterations have been identified during tumor progression, including an amplification of the distal portion of chromosome 6 containing ki-ras and loss of heterozygosity (LOH) in other chromosomal regions. We have demonstrated that stage specific alterations in the expression of genes which are critical regulators of the cell cycle and apoptosis are functionally important in vivo. C3(1)/Tag mice appear useful for testing particular therapies since growth of the mammary tumors can be reduced using chemopreventive agents, cytokines, and an anti-angiogenesis agent.

Heterotopic endochondrial ossification with mixed tumor formation in C3(1)/Tag transgenic mice is associated with elevated TGF-beta1 and BMP-2 expression.

Transgenic mice which express the simian virus 40 large T-antigen (Tag) under the regulatory control of the hormone responsive rat C3(1) gene develop unusual lesions of heterotopic bone growth associated with mixed tumor formation arising from eccrine sweat glands found only in the foot pads of mice, ischiocavernosus muscle adjacent to bulbourethral glands and occasionally the salivary and mammary glands. These lesions are very similar to mixed tumors arising in several types of human cancers. Based upon electron microscopic examination and immunocytochemical analyses of cellular differentiation markers, the mixed proliferative lesions in this transgenic mouse model begin with the Tag-induced proliferation of epithelial and myoepithelial cells. The proliferation of these two types of cells results in hyperplasia and adenomatous transformation of the epithelial component, whereas the proliferating myoepithelial cells undergo metaplasia to form chondrocytes which deposit extracellular matrix, including collagen fibers. Cartilage develops focally between areas of epithelial proliferation and subsequently ossifies through a process of endochondrial bone formation. The metaplasia of myoepithelial cells to chondrocytes appears to require the inductive interaction of factors produced by the closely associated proliferating epithelial cells, including members of the TGF-beta superfamily. We demonstrate that TGF-beta1 protein accumulates in the extracellular matrix of the lesions, whereas RNA in situ hybridization reveals that BMP-2, another strong inducer of heterotopic bone formation, is overexpressed by the proliferating epithelial cells during the development of ectopic bone. The formation of sarcomatous tumors within the mixed tumors appears to be androgen-dependent and more frequent in mice lacking a normal allele of p53. This process of cartilage and bone induction may mimic epithelial-mesenchymal interactions which occur during embryonic bone formation. These transgenic mice may provide new insights into the processes of ectopic endochondrial bone formation associated with mixed tumor formation and serve as a useful model for human heterotopic bone disease.

Amplification of Ki-ras and elevation of MAP kinase activity during mammary tumor progression in C3(1)/SV40 Tag transgenic mice.

We have previously documented that transgenic mice expressing SV40 Tag regulated by the rat prostatic steroid-binding protein C3(1) 5'-flanking region display multistage mammary tumorigenesis. To delineate genetic changes associated with mammary tumor progression, comparative genomic hybridization (CGH) was performed. CGH revealed a consistent gain of the telomeric region of chromosome 6. This region contains the Ki-ras proto-oncogene. Analyses of genomic DNA by Southern blot demonstrated up to 40-fold amplification of the Ki-ras gene. Ki-ras amplification was detected in 12, 46 and 68% of tumors from 4, 5 and 6 month old mice, respectively, whereas no amplifications were found in any preneoplastic mammary tissues. Tumors bearing Ki-ras gene amplification exhibited high levels of Ki-ras RNA and protein. The over-expressed Ki-Ras protein in these tumors appeared functionally active as indicated by the elevated MAP kinase activity. These data demonstrate that while Ki-ras amplification might not be an early event, there is a strong association between Ki-ras amplification and over-expression and mammary tumor progression in this model. This study also shows that CGH is a powerful and useful technique for identifying chromosomal copy number changes during tumor progression, and that this model may provide a predictable in vivo system for studying gene amplification.

Expression of KAI1 in paraffin-embedded normal, hyperplastic and neoplastic prostate and prostate carcinoma cell lines.

Expression of KAI1, a tumor metastasis suppressor gene, was studied with different fixatives in frozen and paraffin-embedded sections of human and rat prostate carcinoma cell lines and human prostate lesions by immunohistochemistry. Immunoreactivity of the membrane antigen in cell lines was associated with known expression levels in these lines and the fixative used. Formalin and paraformaldehyde helped maintain the immunoreactivity of cells. In human prostate, frozen sections revealed diffuse reactivity of the antigen in normal and neoplastic tissues while paraffin-embedded tissues usually showed focal reactivity, although more than 50% of cases with normal epithelium and adenocarcinomas were reactive. In some cases, pretreatment with trypsin enhanced immunoreactivity. Benign prostatic hyperplasia (BPH) showed the most intense diffuse immunoreactivity, which suggested enhanced expression. Prostatic intraepithelial neoplasia (PIN) also often expressed high levels of KAI1. Three of five metastases were reactive but two primaries and their metastases were not. Lymphocytes in primary carcinomas and lymphocytes and germinal center cells in lymph nodes were immunoreactive, while adjacent primary or metastatic prostate adenocarcinoma epithelium was not immunoreactive. Although paraffin-embedded human tissues were not optimal for determining levels of expression of KAI1, they did show immunoreactivity that could have prognostic value and showed the specific cytoplasmic localization of the protein in cells.

Altered expression of transforming growth factor betas during urethral and bulbourethral gland tumor progression in transgenic mice carrying the androgen-responsive C3(1) 5' flanking region fused to SV40 large T antigen.

We demonstrate that targeted expression of SV40 large T antigen (TAg) to the urethral (periurethral) and bulbourethral gland epithelium leads to adenocarcinoma formation in these tissues after 7 months of age, which are extremely rare sites for spontaneous tumor formation in humans. The development of proliferative lesions in the urethral gland predictably follows a temporal course of progression with approximately one third of male animals developing urethral tumors by 1 year of age. Tumor progression in these organs correlates to the level of TAg and p53 expression. Immunoprecipitation confirmed that SV40 TAg protein was bound to p53 and Rb p110 in vivo. Expression of transforming growth factor beta (TGFbetas) was evaluated during tumor progression of urethral gland carcinomas. Elevations of intracellular and extracellular TGFbeta1 and extracellular TGFbeta3 were found in preneoplastic and neoplastic lesions, suggesting that increased TGFbetas may augment tumor growth. c-Met expression showed a tendency for increased expression in the urethral gland carcinomas. We speculate that the directed expression of SV40 TAg by the hormone responsive C3(1) gene and subsequent tumor formation in these organs is influenced by androgens, since these tissues and carcinomas express androgen receptor (AR) and arise only in male transgenic mice. Several cell lines established from the urethral carcinomas were also shown to express AR, but are not androgen dependent in culture. To our knowledge, this is the first transgenic animal model for urethral and bulbourethral carcinomas. This transgenic mouse model and the cell lines derived from it may provide a unique opportunity for dissecting molecular mechanisms involved in the tumorigenesis of these organs which otherwise rarely develop cancer.

Genetic alterations in the development of mammary and prostate cancer in the C3(1)/Tag transgenic mouse model.

We have generated a transgenic mouse model in which female mice develop ductal mammary adenocarcinomas and male mice develop prostatic adenocarcinomas by using a transgene containing the hormone-responsive rat prostatic steroid binding protein 5' flanking region C3(1) fused to the simian virus 40 (SV40) large T antigen. We have identified some genetic alterations during mammary and prostate tumor progression: (i) p53 is functionally inactivated during mammary cancer development without p53 mutations; (ii) Alterations in apoptosis during mammary tumor progression are p53 and bcl-2 independent; (iii) Ha-ras mutations occur early in the development of prostate cancer. This unique animal model offers the opportunity to study multistep tumorigenesis in these organs.

Development and characterization of a mouse prostate adenocarcinoma cell line: ductal formation determined by extracellular matrix.

BACKGROUND: Tumor vaccines show promise as a new approach for treating cancer. We have developed a murine prostate cancer cell line which can be used to study growth factor and extracellular matrix regulation of prostate differentiation and will be useful for generating tumor vaccines using the C3(1)/TAG transgenic model of prostate cancer. METHODS: Pr-14 cells were established in defined growth media (GM) and grown in GM, GM + 2% fetal bovine serum (FBS) or DMEM + 10% FBS on plastic, collagen, or Matrigel. Immunofluorescence and Western blot analyses were performed using antibodies to cytokeratin, vimentin, SV40 large T-antigen, and androgen receptor (AR). RESULTS: Pr-14 cells are cytokeratin-positive, vimentin-negative, and express SV40 large T-antigen. These cells are tumorigenic when injected into athymic nude mice and appear to be androgen-independent. Pr-14 cell lines are nontumorigenic when injected into syngeneic FVB/N mice, but form tumors in transgenic TAG-expressing FVB/N mice. Cell growth and morphology are dependent on media composition which determines whether ductal or acinar structures form when grown on Matrigel. CONCLUSIONS: The mouse prostate adenocarcinoma cell line, Pr-14, undergoes alterations in the state of differentiation dependent upon serum concentration when grown on Matrigel. The Pr-14 cell line is a useful reagent to study prostate cell/extracellular matrix interactions, and for immunotherapy and cancer vaccine studies in C3(1)/TAG transgenic mice.

Reduced p53 dosage associated with mammary tumor metastases in C3(1)/TAG transgenic mice.

Transgenic mice expressing the simian virus 40 large T-antigen (TAG) under the regulatory control of the rat prostatic steroid binding protein C3(1) gene develop mammary carcinomas (in females) and prostate carcinomas (in males). The development of carcinomas occurs several months after the appearance of dysplastic lesions, suggesting that TAG is necessary but insufficient for tumor formation and that other genetic events are involved in this process. TAG is known to bind to p53 and to result in its functional inactivation, which is believed to be a critical step in TAG-induced transformation. We investigated whether the TAG-p53 interaction is rate limiting in the development of phenotypic changes in these transgenic mice by crossing C3(1)/TAG transgenics with mice carrying null mutations of the p53 gene. TAG-expressing animals with a p53+/- genotype developed much more aggressive mammary tumors, as evidenced by increased numbers and size of metastases, than did TAG-expressing animals carrying two wild-type p53 alleles. While p53 was expressed in primary tumors, p53 expression appeared to be reduced or lost in many metastases in mice carrying either the p53+/+ or p53+/- genotypes. The tumorigenic process did not appear to be due to the loss of the second wild-type p53 allele or the gain of dominant oncogenic mutations in p53, as no mutations were detected in either primary or metastatic tumors by polymerase chain reaction--single-strand conformation polymorphism analyses. These findings suggest that despite the presence of TAG, p53 levels influence the characteristics of the late stages of mammary tumor growth and accelerate metastases. Functional loss of p53 and not p53 mutations participates in TAG-induced mammary carcinoma development and progression.

Multiple regulatory regions control the expression of Ets-1 in the developing mouse: vascular expression conferred by intron I.

Ets-1, a developmentally-regulated protooncogene, is expressed in multiple tissues during different stages of mouse development and cellular differentiation including high levels in lymphoid organs and endothelium. The putative roles of this DNA-binding protein in lymphoid development and maturation, as well as in angiogenesis and tumor vascularization, suggest that the regulation of Ets-1 may be critical to understanding these important developmental processes. We have cloned the mouse Ets-1 5' flanking region which shows significant homology to the human 5' flanking region, including potential transcription factor binding sites. Various amounts of mouse Ets-1 5' flanking, exon and intron sequences have been fused to the E. coli lacZ reporter gene and introduced into the mouse germline to identify genomic regions which regulate the developmental and tissue-specific expression of Ets-1. The 2.4 kb 5' flanking region of Ets-1 directs lacZ expression to the folding neural tube of embryos at gestational day 8.5 which is identical to the endogenous expression pattern of Ets-1. However, at later times in gestation, up to 5.3 kb of 5' flanking region results only in aberrant expression and is not able to confer lacZ expression in lymphoid or vascular tissues. When the first exon and 9 kb of the first intron are included with 5' flanking sequences, using an enhancer-trap-strategy, lacZ expression is observed in developing vessels, meninges and choroid plexus which correlates to endogenous Ets-1 expression. Further characterization of the vascular-specific element contained within intron I will provide important insights into the mechanisms controlling gene expression during angiogenesis.

p53-independent apoptosis during mammary tumor progression in C3(1)/SV40 large T antigen transgenic mice: suppression of apoptosis during the transition from preneoplasia to carcinoma.

Alterations in apoptosis and associated mechanisms during mammary tumor progression were investigated in transgenic mice expressing the SV40 large T antigen (T(AG)) driven by the rat prostatic steroid-binding protein C3(1) 5'-flanking region. Apoptosis levels, assessed by in situ end labeling, were low in normal mammary epithelial cells, highest in atypical hyperplasias (preneoplastic lesions), and less pronounced in adenocarcinomas. Preneoplastic cells maintain the ability to undergo apoptosis as a mechanism of tumor growth suppression, but this critical control of apoptosis is lost as these lesions progress to carcinomas. These alterations in apoptosis occur during mammary tumor progression in mice containing wild-type p53+/+ genotype as well as in mice with the p53-/- genotype. Thus, apoptosis in this tumor model occurs through a p53-independent mechanism. Because other studies have demonstrated p53-dependent apoptosis in T(AG)-induced choroid plexus tumors of transgenic mice, we propose that the role of p53 in apoptosis may be tissue-specific. In addition, bcl-2 protein was not expressed in any mammary lesions. SV40 T(AG) expression, which correlated with the nuclear p53 protein at all stages of tumor progression, was low in normal mammary epithelial cells, moderately high in atypical hyperplasias, and strongly expressed in adenocarcinomas. No p53 mutations were found at any stage of mammary adenocarcinoma development, suggesting that tumor progression does not require a dominantly acting p53 mutation in this transgenic model. p2l(Waf1/Cip1), a cyclin-dependent kinase inhibitor, was expressed in normal mammary tissue but was not detected in the mammary carcinomas, despite high nuclear accumulation of wild-type p53 protein, suggesting functional loss of p53 due to binding of SV40 T(AG), to p53. These findings suggest that suppression of apoptosis during the transition from atypical hyperplasia to adenocarcinoma appears to be a critical event for mammary cancer development in C3(1)/T(AG) transgenic mice and occurs by p53- and bcl-2-independent pathways.

The human ETS1 gene: genomic structure, promoter characterization and alternative splicing.

Genomic clones encompassing the human ETS1 gene were isolated and utilized to define its molecular organization. This gene consists of eight exons spanning over 60 kb. The 5' end of the human ETS1 gene was subcloned and characterized. S1 nuclease, primer extension and RNAase protection analyses of human mRNAs showed multiple transcription initiation sites. DNA sequence analysis indicated a high G + C content in the promoter region and the absence of either a 'TATA' box or a 'CAAT' box. Six consensus recognition sequences for the transcription factor SP1, two AP1 consensus sequences and one consensus AP2 recognition sequence were identified, as well as two GC elements with dyad symmetry. A palindromic region similar to the serum response element of the c-fos gene and two octamer consensus recognition sequences were located upstream of the promoter region. A series of promoter deletion constructs positioned upstream from the bacterial chloramphenicol acetyl transferase gene were transfected into HeLa cells and their functional promoter activity assayed. The deletion constructs identified the 5' boundary for maximum promoter activity at 486 bp upstream of the first initiation site and suggest possible positive and negative regulatory regions. Polymerase chain reaction analysis of ETS1 cDNA identified several amplified products, indicating alternative splicing. In addition to the presence of mRNA products lacking exon VII, products lacking exon IV, as well as ones lacking both exons IV and VII, were found.

Molecular organization and differential polyadenylation sites of the human ETS2 gene.

The human ETS2 gene is a member of the ets gene family (ETS1, ETS2, ERG, ELK1 and ELK2) with amino acid similarity to the v-ets oncogene of the avian leukemia virus, E26. The ETS2 gene is composed of 10 exons, nine of which contribute to the open reading frame encoding 469 amino acids. The ETS2 gene directs the synthesis of three RNA transcripts, which differ from each other by the length of their 3' ends. This heterogeneity of 3' end is the major reason for the size differences between the transcripts, presumably reflecting utilization of three different polyadenylation signals/sites. The coding regions of all of the ETS2 RNA species are the same length and, thus, should contain the same open reading frame.

Isoforms of the human ets-1 protein: generation by alternative splicing and differential phosphorylation.

The ets-1 gene belongs to the ets gene family (ets-1, ets-2, erg, and elk) and is homologous to the v-ets oncogene found in the avian leukemia virus E26. The ets-1 gene products were characterized using a specific monoclonal antibody developed against a bacterially expressed v-ets protein. The ets-1 gene product in the human T-cell line CEM was found to consist of at least six species: four major species with apparent molecular weights of 51 kDa (p51), 48 kDa (p48), 42 kDa (p42), and 39 kDa (p39); and two minor species of 52 kDa (pp52) and 49 kDa (pp49), which are demonstrated to be the phosphorylated forms of p51 and p48, respectively. All of the ets-1 proteins are related to each other and are considered products of the ets-1 gene. Subcellular localization showed that the pp52 and p51 are found mainly in the cytoplasm, while p48 and p39 are found mainly in the nucleus. Specific antibodies against various exons of ets-1 showed that both p42 and p39 lack a region corresponding to exon VII. Polymerase chain reaction analyses revealed the presence of an additional RNA product that corresponds to mRNA lacking exon VII. These results suggest that the human ets-1 gene encodes multiple proteins that are generated by at least two distinct mechanisms: alternative splicing of mRNA and protein phosphorylation.

ETS family of genes in leukemia and Down syndrome.

The human ETS2 and ERG genes are members of the ETS gene family, with sequence homology to the viral ets gene of the avian erythroblastosis retrovirus, E26. These genes are located on chromosome 21 and molecular genetic analysis of Down syndrome (DS) patients with partial trisomy 21 suggested that ETS2 may be a gene within the minimal DS genetic region. We have, in fact, been able to confirm the presence of the ETS2 gene dosage in triplicate occurring in occult human 21 chromosome abnormalities. It is known that ERG and ETS2 gene translocations occur in certain specific leukemias associated with defined chromosome rearrangements [e.g., t(8;21)]. Moreover, it is known that DS individuals are at greater risk for leukemic disease than their normal familial cohorts, implying that trisomy of that region of human chromosome 21 may play a role in the development of this type of neoplasia. The human ETS genes, first identified in our laboratory, are highly conserved, being found from lower organisms, like Drosophila and sea urchin, to humans. In mammals, the ETS genes are structurally distinct, located on separate chromosomes; they are transcriptionally active and differentially regulated. The ETS2 protein is phosphorylated and turns over with a half-life of approximately 20 min. After activation with the tumor promoter, TPA, the level of ETS2 elevates 5- to 20-fold. The properties of the ETS2 protein, such as nuclear localization, phosphorylation, rapid turnover, and response to protein kinase C, indicate that this protein belongs to a group of oncogene proteins thought to have regulatory functions in the nucleus. In the mouse thymus ets-1 and ets-2 are 8-10-fold higher, respectively, in the CD4+ subset than in other subsets examined, suggesting a role in T-cell development for these genes. Cells transfected with the cellular ets-2 gene, expressing higher levels of ets-2 products, showed a stimulated proliferation response, abolished their serum requirement and formed colonies in soft agar that could induce tumors in nude mice. Collectively, these data suggest that this family of genes might play a role in controlling specific steps of the signaling transduction pathway. Thus, the ETS genes, as other genes with homology to viral oncogenes, might be instrumental in regulating cellular growth and differentiation, as well as organismal development.

A heat-shock-inducible eukaryotic expression vector.

We describe a construct, pHS/Cla, containing the Drosophila melanogaster hsp70 promoter which serves as an inducible expression vector in mammalian cells. The construct was made in the plasmid pAT153, a derivative of pBR322. In transient transfections of human H9 T-cells, the transactivator of transcription protein of the human AIDS virus HIV-1 was functionally expressed in response to heat shock. The promoter is very tightly regulated in that no expression can be detected at 37 degrees C. Expression is highly induced upon heat shock and lasts at least 4 h. The construct contains a unique ClaI site for cloning and expressing potentially any gene.

A plasmid vector for cloning and expression of gene segments: expression of an HTLV-I envelope gene segment.

We describe a cloning-expression vector system for selecting DNA fragments containing open reading frames (ORFs) and expressing them as beta-galactosidase (beta Gal) hybrid fusion proteins. The plasmid vector, pWS50, utilizes the very strong and easily regulated bacteriophage lambda promoter pL, and the efficient translation initiation signals of the N-terminal segment of the lambda cII gene. Fused distally to and out of translational phase with cII is the E. coli lacZ gene, lacking its own transcriptional and translational initiating signals. A unique restriction enzyme site (NruI) is located between the upstream regulatory sequences and the lacZ gene, which provides a cloning site for the insertion of blunt ended DNA fragments. In addition, there are two other unique restriction sites (NheI and BamHI) located in this region which can also be used as closing sites. If a DNA fragment does not contain any translation termination codons (i.e., an ORF), and is inserted correctly into the vector, the translational reading frame between cII and lacZ can be restored. Colonies containing these recombinants can be easily screened as LacZ+ on lactose indicator media. The beta-galactosidase fusion proteins produced from the LacZ+ recombinants are identified on sodium dodecyl sulfate polyacrylamide gels by their large size and high level of production. To test the ORF cloning-expression system, a segment of the human T-cell lymphotrophic virus type I envelope gene was cloned and expressed at high levels. The envelope-beta Gal fusion protein was recognized by antibodies in serum from a patient with adult T-cell leukemia.