Laboratory Predictors of Hemolytic Anemia in Patients With Systemic Loxoscelism.

OBJECTIVES: To develop a sensitive and specific protocol for detecting preclinical hemolysis in patients with brown recluse spider (BRS) bites by comparing a large cohort of individuals with brown recluse spider (BRS) bites with and without hemolytic anemia. METHODS: A cross-sectional, retrospective analysis of clinical features and laboratory values, including urinalysis (UA) and peripheral blood results, and timing of positive laboratory values prior to a significant drop in hematocrit was performed to evaluate effective predictors of clinically significant hemolysis. RESULTS: In total, 275 patients with BRS bites were identified (64 with hemolytic anemia). Sensitivity and specificity of UA positive for blood (with and without microscopic hematuria) for detecting hemolysis were 72% and 75%, respectively. The combination of elevated serum total bilirubin (TB) and lactate dehydrogenase (LDH) had greater sensitivity (94%) and specificity (91%) for detecting patients developing hemolysis. When TB and LDH were evaluated prior to a significant decrease in hematocrit, they were positive in 82% of cases, while UA was positive for blood prior to a hematocrit decrease in 38% of cases. CONCLUSIONS: Serum TB and LDH levels are more effective at detecting preclinical hemolysis than UA and should be serially analyzed to triage patients with BRS bites before life-threatening hemolysis occurs.

Chronic myeloid leukemia with pure erythroid leukemia blast crisis.

Chronic myeloid leukemia (CML), a myeloproliferative neoplasm defined by the presence of the BCR-ABL1 oncogene created by the reciprocal translocation t(9;22)(q34.1;q11.2), can often be controlled by medications that inhibit this constitutive tyrosine kinase. However, if these therapies fail, the disease may progress to a form resembling an acute leukemia. While most of these CML 'blast crises' are characterized by blasts with a myeloid (granulocytic) or lymphoid lineage, these blasts may rarely be predominantly erythroid. Cases of CML erythroid blast crises have been reported; however, secondary pure erythroid leukemia arising from a CML blast crisis has only been definitively reported once before. We report the second definitive case of pure erythroid leukemia with the t(9;22)(q34.1;q11.2) presenting as a CML blast crisis and characterize the morphologic, immunophenotypic, flow cytometric, cytogenetic, and molecular findings.

Dural Marginal Zone Lymphoma in a Patient With a Hepatitis C Virus Infection.

Primary dural marginal zone lymphomas (MZLs) are exceptionally rare, with fewer than 100 cases reported to date. While the association between hepatitis C virus (HCV) infection and lymphoma is well established, it is unclear if this association extends to all anatomic sites. Here we report a case of dural MZL in a 61-year-old woman with an HCV infection. To our knowledge, this is the first report of a dural MZL associated with an HCV infection in an immunocompetent patient and was successfully treated with radiotherapy and rituximab. As such, future cases of primary MZL found in the dura should prompt consideration of co-infection with microbials such as HCV and upfront treatment with anti-virals should be considered.

Anaplastic Large Cell Lymphoma Manifesting as Pleural Effusion in a Patient with Long-Standing Eosinophilia.

Anaplastic large cell lymphoma (ALCL) is a lymphoma of T-cell origin, characterized by the presence of large lymphoid cells with abundant cytoplasm and pleomorphic, often horseshoe-shaped nuclei (hallmark cells), as well as strong and uniform expression of cluster of differentiation (CD)30. Two distinct clinicopathologic categories of ALCL include primary cutaneous ALCL and systemic ALCL. Systemic ALCL is further classified into anaplastic lymphoma kinase (ALK)-positive, ALK-negative, and breast implant-associated ALCL. Most ALCLs occurring in adults are ALK negative and present in lymph nodes rather than extranodal sites.Primary diagnosis of ALCL in the pleural fluid is extremely rare, with no convincing recent reports available that are based in current understanding of this entity. Herein, we describe a well-characterized case of ALK-negative ALCL with no rearrangement but amplification of DUSP22/IRF4, diagnosed by cytologic examination of the pleural effusion in a 68-year-old white man with a 3-year history of unexplained eosinophilia and pulmonary infiltrates. Also, we present a review of the literature and discuss the current understanding of ALCL based on the 2016 revision of the World Health Organization (WHO) classification of lymphoid neoplasms.

Transcriptional repression of the Neurofibromatosis-1 tumor suppressor by the t(8;21) fusion protein.

Von Recklinghausen's disease is a relatively common familial genetic disorder characterized by inactivating mutations of the Neurofibromatosis-1 (NF1) gene that predisposes these patients to malignancies, including an increased risk for juvenile myelomonocytic leukemia. However, NF1 mutations are not common in acute myeloid leukemia (AML). Given that the RUNX1 transcription factor is the most common target for chromosomal translocations in acute leukemia, we asked if NF1 might be regulated by RUNX1. In reporter assays, RUNX1 activated the NF1 promoter and cooperated with C/EBPalpha and ETS2 to activate the NF1 promoter over 80-fold. Moreover, the t(8;21) fusion protein RUNX1-MTG8 (R/M), which represses RUNX1-regulated genes, actively repressed the NF1 promoter. R/M associated with the NF1 promoter in vivo and repressed endogenous NF1 gene expression. In addition, similar to loss of NF1, R/M expression enhanced the sensitivity of primary myeloid progenitor cells to granulocyte-macrophage colony-stimulating factor. Our results indicate that the NF1 tumor suppressor gene is a direct transcriptional target of RUNX1 and the t(8;21) fusion protein, suggesting that suppression of NF1 expression contributes to the molecular pathogenesis of AML.

Mtgr1 is a transcriptional corepressor that is required for maintenance of the secretory cell lineage in the small intestine.

Two members of the MTG/ETO family of transcriptional corepressors, MTG8 and MTG16, are disrupted by chromosomal translocations in up to 15% of acute myeloid leukemia cases. The third family member, MTGR1, was identified as a factor that associates with the t(8;21) fusion protein RUNX1-MTG8. We demonstrate that Mtgr1 associates with mSin3A, N-CoR, and histone deacetylase 3 and that when tethered to DNA, Mtgr1 represses transcription, suggesting that Mtgr1 also acts as a transcriptional corepressor. To define the biological function of Mtgr1, we created Mtgr1-null mice. These mice are proportionally smaller than their littermates during embryogenesis and throughout their life span but otherwise develop normally. However, these mice display a progressive reduction in the secretory epithelial cell lineage in the small intestine. This is not due to the loss of small intestinal progenitor cells expressing Gfi1, which is required for the formation of goblet and Paneth cells, implying that loss of Mtgr1 impairs the maturation of secretory cells in the small intestine.

Thrombocytopenia in patients with melanoma receiving immune checkpoint inhibitor therapy.

BACKGROUND: Immune checkpoint inhibitors, including antibodies against programmed death 1 (PD-1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4), are being used with increasing frequency for the treatment of cancer. Immune-related adverse events (irAEs) including colitis, dermatitis, and pneumonitis are well described, but less frequent events are now emerging with larger numbers of patients treated. Herein we describe the incidence and spectrum of thrombocytopenia following immune checkpoint inhibitor therapy and two severe cases of idiopathic thrombocytopenic purpura (ITP). CASE PRESENTATIONS: A 47-year-old female with recurrent BRAF mutant positive melanoma received combination anti-PD-1 and anti-CTLA-4. Two weeks later, she presented with mucosal bleeding, petechiae, and thrombocytopenia and was treated with standard therapy for ITP with steroids and intravenous immunoglobulin (IVIG). Her diagnosis was confirmed with bone marrow biopsy, and given the lack of treatment response, she was treated with rituximab. She began to have recovery and stabilization of her platelet count that ultimately allowed her to be retreated with PD-1 inhibition with no further thrombocytopenia. A second patient, a 45-year-old female with a BRAF wild-type melanoma, received anti-PD-1 monotherapy and became thrombocytopenic 43 days later. Three weeks of steroid treatment improved her platelet count, but thrombocytopenia recurred and required additional steroids. She later received anti-CTLA-4 monotherapy and developed severe ITP with intracranial hemorrhage. Her ITP resolved after treatment of prednisone, IVIG, and rituximab and discontinuation of checkpoint inhibition. In a retrospective chart review of 2360 patients with melanoma treated with checkpoint inhibitor therapy, <1% experienced thrombocytopenia following immune checkpoint inhibition, and of these, most had spontaneous resolution and did not require treatment. CONCLUSIONS: Thrombocytopenia, especially ITP, induced by immune checkpoint inhibitors appears to be an uncommon irAE that is manageable with observation in mild cases and/or standard ITP treatment algorithms. In our series, the majority of patients had mild thrombocytopenia that resolved spontaneously or responded to standard corticosteroid regimens. However, in two severe cases, IVIG and rituximab, in addition to steroids, were required. Checkpoint inhibition was resumed successfully in the first patient but rechallenge was not tolerated by the second patient.

Non-Hodgkin's lymphoma involving the liver: clinical and therapeutic considerations.

Primary hepatic non-Hodgkin's lymphoma (NHL) is a rare disease that presents unique diagnostic and therapeutic challenges. Secondary liver involvement by lymphoma is common and can complicate treatment decisions. A review of the published case reports and the few larger series suggests that primary hepatic NHL represents a heterogeneous mixture of disparate diseases rather than a single entity. Presentations vary from the incidental discovery of hepatic abnormalities in an otherwise asymptomatic patient to that of fulminant hepatic failure with rapid progression of encephalopathy to coma and death. The clinical, laboratory, and radiographic characteristics are nonspecific, which means the diagnosis is often not suspected until histopathologic examination of liver tissue. There appears to be a strong association between primary hepatic NHL and the hepatitis C virus. Hepatosplenic T-cell lymphoma has attained its own status as a unique disease, whereas case reports suggest that the spectrum of hepatic lymphoma includes many histologies. Involvement of the liver by lymphoma can compound the difficulty of pursuing aggressive chemotherapy in patients who have a life-threatening illness and impaired metabolism of the most effective drugs. Therapy should be tailored to the individual clinical situation, with consideration of the underlying histology and degree of hepatic insufficiency.

International partnerships to promote quality care: faculty groundwork, student projects, and outcomes.

Global health care is now considered a component of core knowledge for 21st century professional nursing practice. The authors describe laying the groundwork for an international experience and projects resulting from a partnership with a nursing program in the developing country of Guyana. In addition, the outcomes from undergraduate nursing international learning experiences for students and practicing nurses are reported. These outcomes provide support for the integration of study abroad as a strategy for enhancing the undergraduate nursing curriculum and ultimately professional nursing practice during the early years of practice.

miR-31 and miR-17-5p levels change during transformation of follicular lymphoma.

The 30% of patients whose indolent follicular lymphoma transforms to aggressive diffuse large B-cell lymphoma (DLBCL) have poor survival. Reliable predictors of follicular B-cell lymphoma transformation to DLBCL are lacking, and diagnosis of those that will progress is challenging. MicroRNA, which regulates gene expression, has critical functions in the growth and progression of many cancers and contributes to the pathogenesis of lymphoma. Using 5 paired samples from patients who presented with follicular lymphoma and progressed to DLBCL, we identified specific microRNA differentially expressed between the two. Specifically, miR-17-5p levels were low in follicular lymphoma and increased as the disease transformed. In contrast, miR-31 expression was high in follicular lymphoma and decreased as the lymphoma progressed. These results were confirmed in additional unpaired cases of low-grade follicular lymphoma (n = 13) and high-grade follicular lymphoma grade 3 or DLBCL (n = 17). Loss of miR-31 expression in DLBCL was not due to deletion of the locus. Changes in miR-17-5p and miR-31 were not correlated with immunophenotype, genetics, or status of the MYC oncogene. However, increased miR-17-5p expression did significantly correlate with increased expression of p53 protein, which is indicative of mutant TP53. Two pro-proliferative genes, E2F2 and PI3KC2A, were identified as direct messenger RNA targets of miR-31, suggesting that these may contribute to follicular lymphoma transformation. Our results indicate that changes in miR-31 and miR-17-5p reflect the transformation of follicular lymphoma to an aggressive large B-cell lymphoma and may, along with their targets, be viable markers for this process.

Data-Driven Iterative Refinement of Bone Marrow Testing Protocols Leads to Progressive Improvement in Cytogenetic and Molecular Test Utilization.

OBJECTIVES: To determine the effect of iterative refinement of standard ordering protocols on test utilization and results for bone marrow biopsy specimens. METHODS: Eighteen months of test utilization and result data were used to revise the protocols that determine cytogenetic and molecular test selection on bone marrow specimens and then compared with data obtained following protocol revision. RESULTS: Revision of protocols resulted in reduction in total tests and associated charges, due to a decrease in tests both concordant and discordant with the protocols. These reductions only occurred in diseases for which revisions were made and were limited to cases in which reflex testing was performed. There was an increase in the fraction of positive tests, which was also limited to reflex testing. CONCLUSIONS: Data-driven iterative revision of protocols further improves test utilization and performance, while reducing cost. Analysis of testing data can be used to continuously improve test ordering decisions.

Loss of neurofibromatosis-1 and p19(ARF) cooperate to induce a multiple tumor phenotype.

Inactivation of the neurofibromatosis-1 (NF1) gene de-regulates RAS and cooperates with mutation or loss of the p53 tumor suppressor to induce tumorigenesis. p19(ARF) acts upstream of p53 in an oncogene checkpoint to induce apoptosis in response to activated RAS and other factors that stimulate proliferation. Therefore, we bred p19(ARF-/-) to NF1(+/-) mice to determine if loss of these genes collaborates in tumorigenesis. As expected from the embryonic lethality of NF1 null mice, no mice lacking both p19(ARF) and NF1 were born. Unexpectedly, the loss of one allele of NF1 did not greatly shorten the time to tumor formation in a p19(ARF) null background. The tumor types observed were characteristic of p19(ARF) null animals, not those associated with neurofibromatosis or those observed with NF1(+/-)/p53(+/-) mice. However, seven out of 12 animals developed multiple tumors, some with metastases. This multiple tumor phenotype was not previously observed with p19(ARF)-null mice and suggests a distinct form of cooperation between the loss of these tumor suppressors.

Differential gene expression in anaplastic lymphoma kinase-positive and anaplastic lymphoma kinase-negative anaplastic large cell lymphomas.

Anaplastic large cell lymphoma (ALCL) is an aggressive large T- or null-cell lymphoma. Most ALCLs arising in children and young adults express a constitutively active receptor tyrosine kinase, anaplastic lymphoma kinase (ALK). Anaplastic large cell lymphomas lacking ALK are clinically heterogeneous and their pathogenesis is unknown. This study is the first complementary DNA (cDNA) microarray analysis using RNA extracted from tumor tissue (7 ALK+ ALCLs and 7 ALK- ALCLs) to identify genes differentially expressed or shared between the ALK+ and ALK- tumors. Unsupervised hierarchical clustering using the top 11 most statistically significant discriminator cDNAs correctly grouped all ALK+ and ALK- tumors. Hierarchical clustering analysis using the 44 cDNAs with the greatest differential expression between ALK+ and ALK- RNAs grouped 6 of 7 ALK+ ALCLs together and 1 ALK+ ALCL with the ALK- group. In general, ALK+ tumors overexpress genes encoding signal transduction molecules (SYK , LYN , CDC37) and underexpress transcription factor genes (including HOXC6 and HOX A3 ) compared with the ALK- group. Cyclin D3 was overexpressed in the ALK+ group and the cell cycle inhibitor p19INK4D was decreased in the ALK- group, suggesting different mechanisms of promoting G 1 /S transition. Both groups had similar proliferation rates. Genes highly expressed in both ALK- and ALK+ ALCLs included kinases (LCK, protein kinase C, vav2, and NKIAMRE) and antiapoptotic molecules, suggesting possible common pathogenetic mechanisms as well.

MYC cytogenetic status correlates with expression and has prognostic significance in patients with MYC/BCL2 protein double-positive diffuse large B-cell lymphoma.

MYC/BCL2 double-hit lymphoma (DHL), defined by conventional cytogenetic or fluorescence in situ hybridization (FISH) analysis, and MYC/BCL2 double-positive lymphoma (DPL), defined by immunohistochemistry, are associated with a poor prognosis. However, DHL and DPL are not concordant, and it is unclear whether MYC and BCL2 aberrations have prognostic impact in DPL patients. In a cohort of 135 patients diagnosed with large B-cell lymphoma between 2010 and 2014 in whom MYC/8q24 and BCL2/t(14;18)(q32;q21) statuses were assessed by FISH at diagnosis, we evaluated MYC and BCL2 expression by immunohistochemistry. A total of 54 (40%) cases were positive for MYC and BCL2 supporting DPL. Among them, 19 (35%) had MYC rearrangement including 11 DHLs, 12 (22%) had multiple copies of MYC, 19 had no MYC abnormalities, and in 4 cases FISH analysis failed. BCL2 abnormalities were present in 28/54 (52%) cases (20 rearranged and 8 multiple copies). MYC rearrangement correlated with a significantly worse overall survival in DPL (P<0.05), whereas BCL2 genetic status did not correlate with survival (P>0.05). MYC and BCL2 expression by immunohistochemistry correlates with gene status by FISH; however, immunohistochemistry is neither specific nor adequately sensitive to be used as a surrogate for MYC and BCL2 gene status using any cutoff level. In conclusion, MYC rearrangement identifies a subset of patients with DPL who have a significantly worse prognosis. Although immunohistochemical assessment for MYC and BCL2 may be a helpful initial screen to identify higher-risk patients, FISH analysis for MYC remains important for further risk stratification in patients with DPL.

Expression of c-FLIP in classic and nodular lymphocyte-predominant Hodgkin lymphoma.

Different molecular pathways are believed to be involved in the pathogenesis of classic Hodgkin lymphoma as opposed to non-Hodgkin lymphoma. Antiapoptotic mechanisms have been proposed for classic Hodgkin lymphoma, including expression of the cellular Fas-associated death domain-like interleukin-1beta-converting enzyme inhibitory protein (c-FLIP), which plays a critical role in resistance to CD95/Fas-mediated apoptosis. In this study, we compare the expression of c-FLIP in the neoplastic cells of classic Hodgkin lymphoma and nodular lymphocyte-predominant Hodgkin lymphoma cases. Sixteen cases of classic Hodgkin lymphoma and 19 cases of nodular lymphocyte-predominant Hodgkin lymphoma were reviewed. Of 16 classic Hodgkin lymphoma cases, 13 cases (81%) were c-FLIP-positive, compared with 6 of 19 (32%) nodular lymphocyte-predominant Hodgkin lymphoma cases. Strong cytoplasmic staining was seen in 7 of 13 c-FLIP-positive classic Hodgkin lymphoma cases, in contrast with only 2 of 6 c-FLIP-positive nodular lymphocyte-predominant Hodgkin lymphoma cases. The 2 cases of nodular lymphocyte-predominant Hodgkin lymphoma with strong c-FLIP expression were associated with transformation to large B-cell lymphoma. An additional 15 cases of diffuse large B-cell lymphoma were studied for c-FLIP expression. All but 1 were c-FLIP-positive. In conclusion, we detected c-FLIP in a significantly lower proportion of nodular lymphocyte-predominant Hodgkin lymphoma cases compared with classic Hodgkin lymphoma cases. Therefore, c-FLIP expression may not be the major mechanism of pathogenesis in nodular lymphocyte-predominant Hodgkin lymphoma. However, strong c-FLIP expression in nodular lymphocyte-predominant Hodgkin lymphoma was associated with transformation to large B-cell lymphoma in 2 cases. c-FLIP expression is not limited to Hodgkin lymphoma, because the majority of diffuse large B-cell lymphoma cases tested were strongly c-FLIP-positive.

Adult neuroblastoma complicated by increased intracranial pressure: a case report and review of the literature.

Neuroblastoma is the third most commonly occurring malignancy of the pediatric population, although it is extremely rare in the adult population. In adults, neuroblastoma is often metastatic and portends an extremely poor overall survival. Our case report documents metastatic neuroblastoma occurring in a healthy 29-year-old woman whose course was complicated by an unusual presentation of elevated intracranial pressures. The patient was treated with systemic chemotherapy, I(131) metaiodobenzylguanidine (MIBG) radiotherapy, and autologous stem cell transplant (SCT). Unfortunately the patient's response to therapy was limited and she subsequently died. We aim to review neuroblastoma in the context of increased intracranial pressure and the limited data of neuroblastoma occurring in the adult population, along with proposed treatment options.

The ambiguous boundary between EBV-related hemophagocytic lymphohistiocytosis and systemic EBV-driven T cell lymphoproliferative disorder.

Epstein Barr virus (EBV)-related hemophagocytic lymphohistiocytosis (EBV-HLH) is a form of acquired, infection-related HLH which typically represents a fulminant presentation of an acute EBV infection of CD8+ T cells with 30-50% mortality rate. Systemic EBV-positive lymphoproliferative disease of childhood (SE-LPD) is a rare T cell lymphoproliferative disorder predominantly arising in the setting of acute EBV infection, often presenting with HLH. Since both entities have been associated with clonal T cell populations, the discrimination between these diseases is often ambiguous. We report a unique case of a 21 years old female who presented with clinical and laboratory findings of florid HLH in the setting of markedly elevated EBV titers (>1 million) and an aberrant T cell population shown to be clonal by flow cytometry, karyotype, and molecular studies. This case raises the differential of EBV-HLH versus SE-LPD. Review of the literature identified 74 cases of reported EBV-HLH and 21 cases of SE-LPD with associated HLH in 25 studies. Of those cases with available outcome data, 62 of 92 cases (67%) were fatal. Of 60 cases in which molecular clonality was demonstrated, 37 (62%) were fatal, while all 14 cases (100%) demonstrating karyotypic abnormalities were fatal. Given the karyotypic findings in this sentinel case, a diagnosis of SE-LPD was rendered. The overlapping clinical and pathologic findings suggest that EBV-HLH and SE-LPD are a biologic continuum, rather than discrete entities. The most clinically useful marker of mortality was an abnormal karyotype rather than other standards of clonality assessment.

LMO2 induces T-cell leukemia with epigenetic deregulation of CD4.

In this study, we present a remarkable clonal cell line, 32080, derived from a CD2-Lmo2- transgenic T-cell leukemia with differentiation arrest at the transition from the intermediate single positive to double positive stages of T-cell development. We observed that 32080 cells had a striking variegated pattern in CD4 expression. There was cell-to-cell variability, with some cells expressing no CD4 and others expressing high CD4. The two populations were isogenic and yet differed in their rates of apoptosis and sensitivity to glucocorticoid. We sorted the 32080 line for CD4-positive or CD4-negative cells and observed them in culture. After 1 week, both sorted populations showed variegated CD4 expression, like the parental line, showing that the two populations could interconvert. We determined that cell replication was necessary to transit from CD4(+) to CD4(-) and CD4(-) to CD4(+). Lmo2 knockdown decreased CD4 expression, while inhibition of intracellular NOTCH1 or histone deacetylase activity induced CD4 expression. Enforced expression of RUNX1 repressed CD4 expression. We analyzed the CD4 locus by Histone 3 chromatin immunoprecipitation and found silencing marks in the CD4(-) cells and activating marks in the CD4(+) population. The 32080 cell line is a striking model of intermediate single positive to double positive T-cell plasticity and invokes a novel mechanism for LMO2's oncogenic functions.

LIM domain only-2 (LMO2) induces T-cell leukemia by two distinct pathways.

The LMO2 oncogene is deregulated in the majority of human T-cell leukemia cases and in most gene therapy-induced T-cell leukemias. We made transgenic mice with enforced expression of Lmo2 in T-cells by the CD2 promoter/enhancer. These transgenic mice developed highly penetrant T-ALL by two distinct patterns of gene expression: one in which there was concordant activation of Lyl1, Hhex, and Mycn or alternatively, with Notch1 target gene activation. Most strikingly, this gene expression clustering was conserved in human Early T-cell Precursor ALL (ETP-ALL), where LMO2, HHEX, LYL1, and MYCN were most highly expressed. We discovered that HHEX is a direct transcriptional target of LMO2 consistent with its concordant gene expression. Furthermore, conditional inactivation of Hhex in CD2-Lmo2 transgenic mice markedly attenuated T-ALL development, demonstrating that Hhex is a crucial mediator of Lmo2's oncogenic function. The CD2-Lmo2 transgenic mice offer mechanistic insight into concordant oncogene expression and provide a model for the highly treatment-resistant ETP-ALL subtype.

HDAC3 is essential for DNA replication in hematopoietic progenitor cells.

Histone deacetylase 3 (HDAC3) contributes to the regulation of gene expression, chromatin structure, and genomic stability. Because HDAC3 associates with oncoproteins that drive leukemia and lymphoma, we engineered a conditional deletion allele in mice to explore the physiological roles of Hdac3 in hematopoiesis. We used the Vav-Cre transgenic allele to trigger recombination, which yielded a dramatic loss of lymphoid cells, hypocellular bone marrow, and mild anemia. Phenotypic and functional analysis suggested that Hdac3 was required for the formation of the earliest lymphoid progenitor cells in the marrow, but that the marrow contained 3-5 times more multipotent progenitor cells. Hdac3(-/-) stem cells were severely compromised in competitive bone marrow transplantation. In vitro, Hdac3(-/-) stem and progenitor cells failed to proliferate, and most cells remained undifferentiated. Moreover, one-third of the Hdac3(-/-) stem and progenitor cells were in S phase 2 hours after BrdU labeling in vivo, suggesting that these cells were impaired in transit through the S phase. DNA fiber-labeling experiments indicated that Hdac3 was required for efficient DNA replication in hematopoietic stem and progenitor cells. Thus, Hdac3 is required for the passage of hematopoietic stem/progenitor cells through the S phase, for stem cell functions, and for lymphopoiesis.