Morphological and Immunophenotypic Clues to the WHO Categories of Acute Myeloid Leukaemia.

Diagnosis and classification of acute myeloid leukaemia (AML) require cytogenetic and molecular genetic investigation. However, while these evaluations are pending, morphology supplemented by immunophenotyping can provide clues to the diagnosis of specific cytogenetic/genetic categories of AML. Most importantly, acute promyelocytic leukaemia can be diagnosed with a high degree of certainty. However, provisional identification of cases associated with t(8; 21), inv(16), t(1; 22), and NPM1 mutation may also be possible. In addition, transient abnormal myelopoiesis of Down's syndrome can generally be diagnosed morphologically.

Does microgranular variant morphology of acute promyelocytic leukemia independently predict a less favorable outcome compared with classical M3 APL? A joint study of the North American Intergroup and the PETHEMA Group.

Few studies have examined the outcome of large numbers of patients with the microgranular variant (M3V) of acute promyelocytic leukemia (APL) in the all-trans retinoic acid era. Here, the outcome of 155 patients treated with all-trans retinoic acid-based therapy on 3 clinical trials, North American Intergroup protocol I0129 and Programa para el Estudio de la Terapéutica en Hemopatía Maligna protocols LPA96 and LPA99, are reported. The complete remission rate for all 155 patients was 82%, compared with 89% for 748 patients with classical M3 disease. The incidence of the APL differentiation syndrome was 26%, compared with 25% for classical M3 patients, and the early death rate was 13.6% compared with 8.4% for patients with classical M3 morphology. With a median follow-up time among survivors of 7.6 years (range 3.6-14.5), the 5-year overall survival, disease-free survival, and cumulative incidence of relapse for patients with M3V were 70%, 73%, and 24%, respectively. With a median follow-up time among survivors of 7.6 years (range 0.6-14.3), the 5-year overall survival, disease-free survival, and cumulative incidence of relapse among patients with classical M3 morphology were 80% (P = .006 compared with M3V), 81% (P = .07), and 15% (P = .005), respectively. When outcomes were adjusted for the white blood cell count or the relapse risk score, none of these outcomes were significantly different between patients with M3V and classical M3 APL.

Acute promyelocytic leukemia: four distinct patterns by flow cytometry immunophenotyping.

A total of 97 acute promyelocytic leukemia (APL) patients with adequate flow cytometry (FC) data, bone marrow aspirates and presence of t(15;17)/PML-RARA by cytogenetics and/or FISH studies were analyzed for immunophenotypic pattern. Leukemic cells had the following phenotype: CD11b-, CD11c-, CD13+, CD33+, CD45+, CD64+/-, CD117+, and HLA-DR-. A subset of cases showed also an expression of CD2, CD4, CD34, and CD56. Based on the immunophenotype and side scatter properties (SSC), four FC patterns were recognized. The majority of cases represented classical (hypergranular) APL and were characterized by high SSC, positive CD117, lack of CD34, heterogeneous CD13, and bright CD33 (pattern 1). Second most common type, corresponding to the hypogranular (microgranular) variant of APL differed from classical APL by low SSC and frequent co-expression of CD2 and CD34 (pattern 2). Rare cases of APL (pattern 3) showed a mixture of neoplastic cells (low SSC/CD2+/CD13+/CD33+/CD34+/CD117+) and prominent population of benign granulocytes/maturing myeloid precursors (high SSC/CD10+/-/CD16+/-/ CD117-). One case showed two APL populations, one with hypogranular and one with hypergranular characteristics (pattern 4). Apart from a well-known FC pattern of hypergranular APL, we presented less common immunophenotypic variants of APL, which helps to identify an additional group of patients who would benefit from fast confirmatory FISH and/or PCR testing for t(15;17)/PML-RARA.

Clinico-pathological profile of acute promyelocytic leukaemia at Al-Amal Oncology-Haematology Centre, Qatar.

This cases series describes the profile of adult patients with acute promyelocytic leukaemia (APt) at a referral hospital in Qatar. Of 34 acute myeloid leukaemia (AML) cases diagnosed, 11(32%) were classified as APt. Disseminated intravascular coagulation was common at presentation (91%). Severe thrombocytopenia was seen in 73%, leukocytosis in 55% and severe anaemia in 45%. Only 2 patients were of the classic hypergranular type. In the remaining 9 patients, 3 morphological subtypes were recognized: microgranular variant (6 patients), hyperbasophilic (2 patients) and regular nuclear outline M3r (1 patient). Translocation t(15;17) was detected in 63% of cases. APL constitutes a high proportion of AML cases in Qatar, with considerable morphological heterogeneity and a oredominance of APL variants with unfavourable oresenting features.

Genotypic and Phenotypic Characteristics of Acute Promyelocytic Leukemia Translocation Variants.

Acute promyelocytic leukemia (APL) is a special disease entity of acute myeloid leukemia (AML). The clinical use of all-trans retinoic acid (ATRA) has transformed APL into the most curable form of AML. The majority of APL cases are characterized by the fusion gene PML-RARA. Although the PML-RARA fusion gene can be detected in almost all APL cases, translocation variants of APL have been reported. To date, this is the most comprehensive review of these translocations, discussing 15 different variants. Reviewed genes involved in APL variants include: ZBTB16, NPM, NuMA, STAT5b, PRKAR1A, FIP1L1, BCOR, NABP1, TBLR1, GTF2I, IRF2BP2, FNDC3B, ADAMDTS17, STAT3, and TFG. The genotypic and phenotypic features of APL translocations are summarized. All reported studies were either case reports or case series indicating the rarity of these entities and limiting the ability to drive conclusions regarding their characteristics. However, reported variants have shown variable clinical and morphological features, with diverse responsiveness to ATRA.

Gene expression profiling of acute promyelocytic leukaemia identifies two subtypes mainly associated with flt3 mutational status.

Acute promyelocytic leukaemia (APL) is a well-defined disease characterized by a typical morphology of leukaemic cells, the presence of t(15;17) translocation and the unique sensitivity to the differentiating effect of all-trans retinoic acid. Nevertheless, some aspects are variable among APL patients, with differences substantially related to morphological variants, peripheral leukocytes count, the presence of a disseminated intravascular coagulopathy, different PML/RARalpha isoforms (long, variable or short) and Fms-like tyrosine kinase 3 (Flt3) mutations. In order to better define this variability, we investigated the gene expression profiles of 18 APL cases revealing, besides a high uniformity in gene expression pattern, the presence of few robust differences among patients able to identify, by an unsupervised analysis, two major clusters of patients characterized by different phenotypes (hypogranular M3v vs classical M3) and by the presence or absence of Flt3 internal tandem duplications (ITDs). Further supervised analysis confirmed that Flt3 status was the APL parameter best associated with these two subgroups. We identified, between Flt3 wild-type and Flt3-ITDs subsets, 147 differentially expressed genes that were involved in the cytoskeleton organization, in the cell adhesion and migration, in the proliferation and the coagulation/inflammation pathways as well as in differentiation and myeloid granules constitution suggesting a role of Flt3 mutations in the pathogenesis and clinical manifestations of APL.

Optimizing treatment for elderly patients with acute promyelocytic leukemia: is it time to replace chemotherapy with all-trans retinoic acid and arsenic trioxide?

This review focuses on the treatment of acute promyelocytic leukemia (APL) in elderly patients and offers recommendations for improving outcomes. Nineteen percent of patients with APL are > or =60 years. Rates of response and survival are lower in elderly compared with younger patients, owing to a higher incidence of early deaths or deaths in remission. However, relapse-free survival rates are similar in both groups. Ongoing trials assess the role of reduced-intensity regimens. All-trans retinoic acid (ATRA) and concurrent arsenic trioxide is associated with high rates of response and molecular remission and low rates of induction deaths. We propose this combination as the treatment of choice in patients with APL, including the elderly. Patients with elevated leukocyte counts may also benefit from gemtuzumab ozogamicin therapy, with or without leukapheresis. Monitoring major organ function and toxicity is essential. Patients should be assessed for minimal residual disease using polymerase chain reaction testing for promyelocytic leukemia-retinoic acid receptor alpha. If molecular relapse is evident, treatment with ATRA and idarubicin, with or without gemtuzumab ozogamicin, is recommended.

[Immunophenotypic Analysis of Acute Promyelocytic Leukemia].

OBJECTIVE: To investigate the immunophenotype of leukemia promyelocytes (LP) in bone marrow of patients with acute promyelocytic leukemia (APL) and to explore their characteristics and significance. METHODS: The immunophenotypes of leukemia cells in 43 patients with APL were analyzed by means of 4 color immunophenotypes; the cell population in which CD45 strength localized at 10(2) and the SSC strength locatized at 10(2) was defined as R3, the cell population in which CD45 strength localized at 10(3) and the SSC strength localized at 10(2) was defined as R5, moreover the ratio of positive cells >80% was defined as strong positive expression, the ratio of positive cells between 20%-80% was difined as weak positive expression, the ratio of positive cells <20% was difined as negative by gating method of CD45/SSC. RESULTS: There was a abnormal cell population (R3) in 79.07% cases; the immunophenotypes of R3 was cheracteried by high SSC, weaker expression of CD45, the rate of CD38, CD9 and CD13 all was 100%, moreover their bright expression (>80%) was 86.05%, 90.70% and 86.05%, respectively; the positive expression rate of CD33, CD117 and CD64 was 97.67%, 95.35% and 83.80% respectively, moreover thier bright expression was 84.04%, 69.77% and 30.23% respectively; the CD15 was weakly expressed in 39.53% cases, the CD34 and HLA-DR were weakly expression in 16.28% and 6.98% cases respectively. All the cases did not express CD116. There were 2 cell populations (R3 and R5) in 20.93% cases, the immunophenotypic features of R3 were cosistant with above mentioning, while the immunophenotypes of R5 were lower than those of R3 SSC; the fluorescence intensity of CD45 was higher, but lower than that in normal lymphycytes, the positive rate of CD9, CD13, MPO was 100%, moreover thier fluorescence intensity was high; they did not expressed CD123, CD25, CD22, CD4, CD64 and CD14. Thereby it can be concluded that the typical immunophenotypes is characterized by CD13(+) CD9(+) CD38(+) CD33(+) CD117(+) CD64(+) CD11b(-) CD34(-) HLA-DR(-) in APL. There was a special immunophenotype in the APL with basophilic granules. Conclusoin: APL has a characteristic immunophenotypic profile, whose typical immunophenotype is characterized by CD13(+) CD9(+) CD38(+) CD33(+) CD117(+) CD64(+) CD11b(-) CD34(-) HLA-DR(-). The special immunophenotype exists in the APL with basophilic granules. Flow cytometric immunophenotyping may be a useful for rapid recognition of APL and has significant for prognosis.

Acute Lymphoblastic Leukemia in Adults - an Analysis of 51 Cases from a Tertiary Care Center in Pakistan.

BACKGROUND: Acute lymphoblastic leukemia (ALL) is a malignant disease in which early lymphoid precursors proliferate and replace the normal hematopoiesis. It has distinctive clinical and biological features. In respect to adult ALL, available data from Pakistan are limited. Therefore we reviewed the demographical and clinico- hematological profiles along with FAB stratification of adult patients with ALL presented at our hospital. MATERIALS AND METHODS: In this cross sectional study, 51 adults (≥15 years) patients with ALL were enrolled from January 2010 to December 2014. RESULTS: The mean age was 23.8±12.9 years with the median age of 18.0 years. The male to female ratio was 2:1. The major complaints were fever (60.7%), generalized weakness (47.0%), overt bleeding (19.6%) and weight loss (13.7%). Physical examination revealed lymphodenopathy as a predominant finding detected in 43.1% followed by splenomegaly and hepatomegaly in 23.5% and 21.5%, respectively. The mean hemoglobin level was 9.0±2.75g/dl with a mean MCV of 82.2±15.4 fl, a mean total leukocyte count of 31.1±64.0x109/l, a mean ANC of 2.1±3.0 x109/l and a mean platelet count of 71.7±85.7x109/l. According to FAB classification, 47.1% were L1 type, 45.1% L2 and 7.8% L3 variant. CONCLUSIONS: Clinico-pathological features appeared comparable to published data. Febrile illness associated with lymphodenopathy was the commonest presentation. FAB classification revealed a predominance of ALL-L1 variant in Pakistani adult patients with ALL.

Acute promyelocytic leukemia in children: experience of the Italian Pediatric Hematology and Oncology Group (AIEOP).

Scanty information is available on acute promyelocytic leukemia (APL) in children, and whether differences are present with respect to the adult form. The experience of the Italian Pediatric Hematology and Oncology Group (AIEOP) will be presented with respect to the following aspects: 1. Incidence of APL. The incidence of APL is generally considered to account for 3-9% of acute myelogenous leukemia (AML) in children and approximately 10-15% in adults. Recently a single Italian pediatric institution reported that APL constituted one third of observed acute nonlymphocytic leukemia (AnLL) cases. Data from the AIEOP cooperative study group have confirmed that APL in Italy is more frequently observed in children as compared to other countries. Environmental and/or genetic factors should be considered to explain such differences. 2. Diagnosis of M3v. The clinical and biological features of the largest series of childhood M3v will be presented and the problems encountered in the proper separation of 'classic' M3 and M3v in children will be discussed. 3. Clinical Aspects. The clinical features of the APL patients enrolled in the AIEOP study groups since 1989, will be presented with emphasis on the recent experience with the use of all-trans retinoic acid. 4. Analysis of PML/RAR alpha Fusion Transcripts. An RT-PCR analysis of 32 pediatric APL cases from cryopreserved bone marrow samples has been performed. It is concluded that APL in children did not differ significantly from the adult form, with the exception of a higher incidence of PML bcr3 breakpoint.

Childhood acute promyelocytic leukemia: a rare variant of nonlymphoid leukemia with distinctive clinical and biologic features.

Of 251 consecutive cases of childhood acute nonlymphocytic leukemia (ANLL) seen at St. Jude Children's Research Hospital over a 12-year period, 16 (6.4%) were classified as promyelocytic according to the French-American-British definition. Patients with this form of leukemia were older at diagnosis than the group representing all other ANLL subtypes (median age, 14.8 vs. 9.0 years); they had lower leukocyte counts (median, 4.5 vs. 25.9 x 10(9)/liter), and a higher percentage were girls (68% vs. 44%). They also were much more likely to have a coagulation abnormality (75% vs. 13%). Only 44% of the promyelocytic group achieved complete remission, compared with 79% of the remaining patients (p = 0.001); however, after a median follow-up of 3.5 years, all but two of the responding patients with promyelocytic leukemia remain in complete remission. The majority of induction failures in the promyelocytic group (six of nine) resulted from complications that developed during periods of marrow hypoplasia or before hypoplasia was induced; whereas in the comparison group, more than half of the patients who failed had evidence of absolute or relative drug resistance. It is concluded that acute promyelocytic leukemia in children differs sufficiently from other subtypes of childhood ANLL to justify clinical trials of selective therapy. Recommendations for the use of heparin and blood component support in these patients are given.

Short-term treatment for adult hypergranular and microgranular acute promyelocytic leukemia.

A high hemorrhagic risk and a complete response to the differentiative agent all-trans-retinoic acid (ATRA) are the main clinical features of acute promyelocytic leukemia (APL), two distinct subtypes of which have been recognized, the common hypergranular leukopenic form (M3) and a microgranular hyperleukocytic variant (M3v). We analyzed, with emphasis on both disease- and therapy-related prognostic factors, the results from a 9-year trial in 65 adults with M3 and M3v APL, treated homogenously with a short-term therapy (STT) program excluding maintenance. STT comprised a maximum of six courses with doxorubicin, cytosine arabinoside (ara-C), and 6-thioguanine. Sixty-five APL patients formed the study group, M3v accounting for 25% of cases. In M3v, the absolute blast cell count was significantly higher (p < 0.0001) and early hemorrhagic deaths were more frequent (p = 0.05). The blast count correlated inversely with the probability of remission (p = 0.005), poor-risk patients being those with > 10 x 10(9)/l blast cells. During the study, the median survival improved from 0.1 to 2.7 years (p = < 0.005). In first place, response to chemotherapy increased from 42 to 84% (p = 0.006), by giving daily prophylactic platelet transfusions (to > 30 x 10(9)/l) and no heparin (course I), and by avoiding too toxic high-dose ara-C and deferring treatment in infected/neutropenic patients showing the atypical differentiative bone marrow pattern (course II). Secondly, the probability of first unmaintained remission differed significantly between patients given intentionally more than four total chemotherapy courses or intermediate/high-dose ara-C consolidation (0.59 at 5 years) and those treated less intensively (0.21) (p < 0.005). Intensive STT was very effective for the management of adult APL patients at standard hemorrhagic risk and receiving optimal supportive care. In high-risk patients with hyperleukocytosis and M3v, induction results could be improved by the concomitant use of ATRA. M3v in adults must be recognized promptly because of the very high early hemorrhagic risk.

Molecular pathogenesis of acute promyelocytic leukaemia and APL variants.

It has been 12 years since the simultaneous discovery of the unique sensitivity of acute promyelocytic leukaemia (APL) to differentiation therapy with all-trans retinoic acid (ATRA) and the discovery that the retinoic acid receptor alpha (RARalpha) gene was rearranged in APL. Nearly 98% of cases of APL are associated with t(15;17) chromosomal translocation and fusion of the PML gene to that encoding RARalpha to yield an abnormal receptor with the capability of de-regulating gene expression in the haematopoietic cell, causing differentiation block and eventually the development of leukaemia. Since this original discovery, four other translocations were described in APL. In each of these the RARalpha gene is fused to different partner genes, all yielding aberrant nuclear receptors. These fusion proteins share in common the ability to repress rather than activate retinoic acid targets, one so strongly that the result is an ATRA-resistant form of the disease. In addition each of the partner proteins is important for normal cell growth and development. In this chapter we explore the biology of the RARalpha, the fusion proteins created in APL and the normal forms of the partner proteins. Through continued study of this disease it is hoped that novel treatments, potentially more applicable to other forms of leukaemia, may arise.

The epidemiology of acute promyelocytic leukaemia.

Recent progress has demonstrated that acute myelogenous leukaemia (AML) can be classified by chromosomal aberrations and leukaemia-specific molecular gene rearrangements into homogeneous biological subgroups. However, descriptive epidemiological reports on AML consider the disease as a single entity. Acute promyelocytic leukaemia (APL) is an example of a truly unique AML subtype that has an easy-to-recognize morphology associated uniformly with distinct chromosomal and gene rearrangement aberration. Thus, APL is amenable to epidemiological studies as a model of human AML with a specific and well-characterized chromosomal and molecular abnormality. This chapter shows that epidemiological characteristics of APL are different from those of non-APL AML using data from the Los Angeles tumour registry and other sources. The principal distinct APL epidemiological features that so far have been described are the constant incidence with age after age 20, equal incidence in males and females and higher frequency among patients originating in Latin America. The APL-specific PML/RARalpha gene rearrangement is different in Latinos and non-Latinos. Therapy-related APL has the same response to treatment and outcome as de novo APL. It is therefore likely that aetiological factors for APL are different from those of other AML subtypes. So far no environmental and/or occupational risk factors have been found for APL. Future molecular studies of the APL-specific fusion gene combined with epidemiological and environmental investigations might lead to better understanding of specific aetiological factors in APL patients.

Usefulness and limitations of serum and urine lysozyme levels in the classification of acute myeloid leukemia: an analysis of 208 cases.

The revised French-American-British (FAB) classification system for acute myeloid leukemia (AML) recommends the determination of serum lysozyme (SL) or urine lysozyme (UL) levels as an aid in distinguishing acute myeloblastic leukemia with maturation (FAB M2) from acute myelomonocytic leukemia (M4). We reviewed retrospectively 208 cases of adult leukemia in which SL and/or UL were obtained. Elevated lysozyme levels were not found in any of the M0, M3, or M7 cases, but were increased (false positive) in three (14%) M1 cases, 18 (19%) M2 cases and one (20%) M6 case. Although a UL value in excess of 3x normal was found in most cases of AML M4 and M5, only five (11%) M4 cases and three (20%) M5 cases had SL elevations of this magnitude. Lysozyme levels need to be interpreted in conjunction with other parameters for FAB classification.

The immunophenotype of adult acute myeloid leukemia: high frequency of lymphoid antigen expression and comparison of immunophenotype, French-American-British classification, and karyotypic abnormalities.

Immunophenotyping has become common in the diagnosis and classification of acute leukemias and is particularly important in the proper identification of cases of minimally differentiated acute myeloid leukemia (AML-M0). To evaluate the immunophenotype of adult AML, 106 cases were studied by cytochemical analysis and by flow cytometry with a panel of 22 antibodies. The results were compared with the French-American-British (FAB) Cooperative Group classification, as well as with available cytogenetic data on each case. CD45, CD33, and CD13 were the most commonly expressed antigens (97.2%, 95.3%, and 94.3%, respectively). Lymphoid-associated antigens were expressed in 48.1% of cases. CD20 was the most commonly expressed lymphoid antigen (17%), although often expressed in only a subpopulation of leukemic cells, followed by CD7 (16%), CD19 (9.8%), CD2 (7.5%), CD3 (6.7%), CD5 (4.8%), and CD10 (2.9%). Some immunophenotypes correlated with FAB type, including increased frequency of CD2 expression in AML-M3; lack of CD4, CD11c, CD36, CD117, and HLA-DR expression in AML-M3; increased frequency of CD20 and CD36 expression and lack of CD34 expression in AML-M5; increased frequency of CD5 expression in AML-M5a; and increased frequency of CD14 expression in AML-M5b, when compared with all other AMLs (P < .05). When compared with AML-M5b, AML-M5a demonstrated a lack of CD4 expression and a high frequency of CD117 expression. Complete morphologic and cytogenetic agreement between AML-M3 and t(15;17) was present, and four of five cases of AML-M4Eo demonstrated inv(16). The remaining case of M4Eo was characterized by a 6;9 translocation, and two other inv(16) cases were not classified as M4Eo. Expression of CD2 was present in two t(15;17) cases and in one inv(16) case, but expression of this antigen was not restricted to AML cases with these karyotypic abnormalities. Similarly, expression of CD19 was not specific for t(8;21) AML. All t(8;21) leukemias demonstrated M2 morphology. With the exception of M3, M4Eo, and a subgroup of M2 leukemias, the FAB classification does not appear to define cytogenetically distinct disease groups in adult AML. Immunophenotypically distinct profiles were identified in the M3 and M5 morphologic groups of the FAB classification. Immunophenotyping studies are helpful in the determination of myeloid lineage. In general, however, they are not sufficiently specific alone to be useful in precisely identifying either FAB or cytogenetically defined disease subtypes.

Acute promyelocytic leukemia toluidine blue subtype.

In the hypergranular group of acute promyelocytic leukemia (APL) a rare subvariant with basophilic granules, metachromatic for toluidine blue, is recognizable. To evaluate the incidence as well as the biological and clinical significance of this subtype, we studied 53 consecutive untreated patients with APL with morphological, cytochemical, immunological and cytogenetic methods. In 10 cases (19% of the total) granules stained metachromatically in percentages of promyelocytes ranging from 16 to 60. In these cases peroxidase positivity was weaker than in the classic hypergranular and microgranular M3 and activities of esterases were usually present; at the ultrastructural level granules contained particulate material. Immunophenotypic and cytogenetic characteristics seemed not to differ from those of other M3 cases. Coagulopathy was usually life-threatening, notwithstanding the low white cell count, and the median survival was short. Hyperhistaminemia-related symptoms were not observed. Cytochemical, immunologic and cytogenetic findings are useful to differentiate this form from M2 with basophilic differentiation and from mast cell leukemia.

Classification of acute myeloid leukaemia in trephine biopsies with special reference to lactoferrin.

In spite of advances in immunohistochemical techniques, the histological subclassification of acute myeloid leukaemia (AML) in bone marrow biopsy has remained difficult. In particular, the translation of the diagnostic criteria of the French-American-British (FAB) cooperative group as primarily defined by bone marrow cytology into histology poses considerable problems. In this study, we investigated the expression of lactoferrin (LF) in various subtypes of AML and studied the usefulness of its immunohistochemical detection combined with a panel of antibodies directed against myeloperoxidase (MPOX), lysozyme (LYS), CD34 and naphthol-AS-D-chloroacetate esterase (NACE) staining in solving this problem. Trephine biopsies of 52 cases of AML were selected for histological evaluation in comparison to bone marrow aspirates classified according to FAB (M1 n = 10, M2 n = 7, M3 n = 11, M4 n = 13 and M5 n = 11). The results obtained confirmed the specificity of LF as a marker for secondary granules in neutrophilic myeloid cells and as a tool to subclassify AML. Its parallel application with (immuno-)staining for MPOX, LYS and NACE has allowed the identification of M1, M3, M5 cases, where there LF is lacking. Typically M2 is characterized by a subpopulation of LF-positive cells which tend to display a myelocytic differentiation. However, M4 shows a heterogeneous expression pattern of LF: M4a may be defined as more immature variant without LF expression while in M4b a more mature myeloid subpopulation stains positive for LF.