Diagnostic workup of acute leukemias of ambiguous lineage.

Acute leukemias of ambiguous lineage (ALAL) comprise acute undifferentiated leukemias (AUL) and mixed-phenotype acute leukemias (MPAL). In the revised fourth edition of the World Health Organization (WHO) classification provided further refinements to the diagnostic criteria for ALAL. Molecular characterization of MPALs using comprehensive next-generation sequencing (NGS) has provided insights into their underlying biology and enabled a deeper understanding of ALAL classification. This review addresses the various components of pathologic assessment to establish a diagnosis of ALAL, and to further subclassify individual cases as AUL or MPAL, with an emphasis on the most up-to-date revisions to diagnostic criteria. In addition, key issues related to the detection of minimal residual disease (MRD) in ALALs and MPALs, and recently uncovered novel molecular diagnostic findings that may be helpful in better distinguishing various types of MPALs from each other, and from their "non-mixed" phenotypic correlates, are also discussed.

Multiparameter flow cytometry applications in the diagnosis of mixed phenotype acute leukemia.

Mixed phenotype acute leukemias (MPALs) represent a rare subgroup of acute leukemias with a poor prognosis. Proper diagnosis and classification of MPAL is extremely important for patients' outcome. Morphology and flow cytometry recognize two types of MPAL: the "bilineal" MPAL with the coexistence of two blast populations of different lineage and truly "biphenotypic" MPAL coexpressing markers of more than one lineage in a homogenous blast population, respectively. The WHO 2008 classification further delineated three categories: associated with t(9;22)/BCR-ABL1 fusion gene, associated with KMT2A (mixed lineage leukemia) rearrangements, and nonotherwise specified. These categories remained unchanged in the WHO2016 update. Molecular studies have further underlined the heterogeneity of MPAL. In this review, rules for the correct assignment of acute leukemia to the MPAL category are discussed, including both flow cytometry and immunohistochemistry on bone marrow or other tissues biopsies. Comparison of the immunophenotypic classification proposals is provided outlining the explorations mandatory for definitive diagnosis. An extensive review of published data summarizes the reported cytogenetic and molecular anomalies. New developments in the understanding of the early stages of hematopoiesis provide clues to the possible etiopathology of these diseases. Finally, current treatment recommendations are summarized and referenced for clinical use, pointing out that allogeneic hematopoietic stem cell transplantation at an early stage should be considered (at least in adult patients). © 2019 International Clinical Cytometry Society.

The genetic basis and cell of origin of mixed phenotype acute leukaemia.

Mixed phenotype acute leukaemia (MPAL) is a high-risk subtype of leukaemia with myeloid and lymphoid features, limited genetic characterization, and a lack of consensus regarding appropriate therapy. Here we show that the two principal subtypes of MPAL, T/myeloid (T/M) and B/myeloid (B/M), are genetically distinct. Rearrangement of ZNF384 is common in B/M MPAL, and biallelic WT1 alterations are common in T/M MPAL, which shares genomic features with early T-cell precursor acute lymphoblastic leukaemia. We show that the intratumoral immunophenotypic heterogeneity characteristic of MPAL is independent of somatic genetic variation, that founding lesions arise in primitive haematopoietic progenitors, and that individual phenotypic subpopulations can reconstitute the immunophenotypic diversity in vivo. These findings indicate that the cell of origin and founding lesions, rather than an accumulation of distinct genomic alterations, prime tumour cells for lineage promiscuity. Moreover, these findings position MPAL in the spectrum of immature leukaemias and provide a genetically informed framework for future clinical trials of potential treatments for MPAL.

Mixed phenotype acute leukemia: A study of 61 cases using World Health Organization and European Group for the Immunological Classification of Leukaemias criteria.

OBJECTIVES: The 2008 World Health Organization (WHO) classification system grouped bilineal and biphenotypic acute leukemias together under a new heading of mixed phenotype acute leukemia (MPAL). The lineage-specific marker criteria have also changed for a diagnosis of MPAL. The goal of this study was to characterize clinical significance of this new group. METHODS: Sixty-one patients diagnosed with MPAL using either European Group for the Immunological Classification of Leukaemias (EGIL) criteria or 2008 WHO criteria were included in this study. RESULTS: Sixteen patients (26%) diagnosed with acute biphenotypic leukemia using EGIL criteria did not fulfill 2008 WHO criteria for MPAL. Cytogenetic data were available for 32 patients, and the most common abnormality was t(9;22) (five of 32 cases). Clinical outcome data suggested that younger patients with MPAL (≤21 years) had better overall survival (OS) in both the EGIL and WHO groups (EGIL, P = .0403; WHO, P = .0601). Compared with 177 patients with acute myeloid leukemia (AML), MPAL patients had better OS (P = .0003) and progression-free survival (P = .0001). However, no difference in OS between MPAL and 387 patients with acute lymphoblastic leukemia was present (P = .599). CONCLUSIONS: As defined by the 2008 WHO classification, fewer patients are now classified as having MPAL than with the EGIL criteria. In this study, patients with MPAL have a better clinical outcome compared with patients with AML.

Lineage classification of childhood acute lymphoblastic leukemia according to the EGIL recommendations: results of the ALL-BFM 2000 trial.

BACKGROUND: Flow cytometry immunophenotyping (FCM) is an undispensable tool for the diagnosis and for the treatment stratification of childhood acute lymphoblastic leukemia. The correlation of the EGIL-classification with prognostically relevant parameters like age, prednisone response and risk group is analyzed. PATIENTS: Between March 2000 and June 2009 12 patients less than 1 year of age, 1 836 patients with 1 to less than 6 years, 620 patients with 6 to less than 10 years, 615 patients with 10 to less than 15 years and 275 patients with 15 to less than 19 years were analyzed with a comprehensive 4-color antibody panel and classified according to the EGIL recommendations. METHODS: Bone marrow or peripheral blood mononuclear cells were isolated by ficoll gradient centrifugation, washed and stained with fluorochrome-conjugated antigen-specific monoclonal antibodies. Cell preparations were acquired and analyzed on a flow cytometer. RESULTS: Centralized FCM was performed for 2 775 patients (82.6%) with B-cell precursor acute lymphoblastic leukemia, 493 patients (14.7%) with T-cell acute lymphoblastic leukemia and 90 patients (2,7%) with biphenotypic acute leukemia. There was a slight overall predominance of male (56.1%) over female (43.9%) patients. Patients with B-cell precursor ALL had a slightly more favourable outcome with a 10 y pEFS of 78 ± 1.0%, compared to patients with a T-ALL or BAL (biphenotypic acute leukemia) phenotype with a 10 y pEFS of 74 ± 1.8% (n.s.) or 69 ± 9.0% (p<0.009), respectively. CONCLUSIONS: FCM according to the EGIL recommendations not only provides diagnostic lineage determination and subclassification but also enables an initial prognostic orientation before MRD (minimal residual disease)-based risk stratification becomes available.

Clinical, immunophenotypic, cytogenetic, and molecular genetic features in 117 adult patients with mixed-phenotype acute leukemia defined by WHO-2008 classification.

Among 4,780 consecutive adult acute lymphoblastic/myeloblastic leukemia patients, we identified 117 (2.4%) patients with mixed-phenotype acute leukemia fulfilling WHO 2008 criteria; these were classified as: Blymphoid+ myeloid (n=64), T-lymphoid+myeloid (n=38), B+T-lymphoid (n=14) and trilineage (n=1). Of 92 patients karyotyped, 59 were abnormal and were classified as: complex (22 of 92), t(9;22)(q34;q11) (14 of 92), monosomy 7 (7 of 92), polysomy 21 (7 of 92), t(v;11q23) (4 of 92), t(10;11)(p15;q21) (3 of 92), while STIL-TAL1 fusion was detected in one (T+My) patient. After investigating common acute leukemia-related mutations in 17 genes, 12 of 31 (39%) patients were found to have at least one mutation, classified with: IKZF1 deletion (4 of 31), and EZH2 (3 of 31), ASXL1 (3 of 31), ETV6 (2 of 31), NOTCH1 (1 of 31), and TET2 (1 of 31) mutations. Array-CGH revealed genomic deletions of CDKN2A (4 of 12), IKZF1 (3 of 12), MEF2C (2 of 12), BTG1 (2 of 12), together with BCOR, EBF1, K-RAS, LEF1, MBNL1, PBX3, and RUNX1 (one of 12 each). Our results indicate that mixed-phenotype acute leukemia is a complex entity with heterogeneous clinical, immunophenotypic, cytogenetic, and molecular genetic features.

Acute leukemias of ambiguous lineage.

The 2008 edition of the WHO Classification of Tumors of Haematopoietic and Lymphoid Tissues recognizes a special category called "leukemias of ambiguous lineage." The vast majority of these rare leukemias are classified as mixed phenotype acute leukemia (MPAL), although acute undifferentiated leukemias and natural killer lymphoblastic leukemias are also included. The major immunophenotypic markers used by the WHO 2008 to determine the lineage for these proliferations are myeloperoxidase, CD19, and cytoplasmic CD3. However, extensive immunophenotyping is necessary to confirm that the cells indeed belong to 2 different lineages or coexpress differentiation antigens of more than 1 lineage. Specific subsets of MPAL are defined by chromosomal anomalies such as the t(9;22) Philadelphia chromosome BCR-ABL1 or involvement of the MLL gene on chromosome 11q23. Other MPAL are divided into B/myeloid NOS, T/myeloid NOS, B/T NOS, and B/T/myeloid NOS. MPAL are usually of dire prognosis, respond variably to chemotherapy of acute lymphoblastic or acute myeloblastic type, and benefit most from rapid allogeneic hematopoietic stem cell transplantation.

[Study on the clinical characteristics of 32 patients with mixed phenotype acute leukemia].

OBJECTIVE: To investigate the clinical and biological characteristics and prognosis of mixed phenotype acute leukemia (MPAL). METHODS: Thirty two patients were diagnosed as MPAL by bone marrow examination, immunophenotyping, cytogenetic and molecular assay and were treated with combined chemotherapy regimens for both acute lymphoblastic and acute myeloid leukemia. Two cases were received allogeneic hematopoietic stem cell transplantation (allo-HSCT). RESULTS: (1) The incidence of MPAL in acute leukemias was 2.6%. There were 16 cases (50.0%) of mixed myeloid and B-lymphoid (M/B), 14(43.8%) myeloid and T-lymphoid (M/T), one each (3.1%) of trilineage (M/B/T) and B- and T-lymphoid (B/T) phenotype. (2) The positive rates of CD34 and HLA-DR were 87.5% and 62.5%, respectively. (3) Abnormal karyotypes were detected in 70.0% of 30 MPAL patients, which were structural and numerical abnormalities including t(9;22), 11q23 and complex karyotypes. (4) The total complete remission (CR) rate was 75.0% and the overall survival (OS) and disease-free survival (DFS) at 2 years were 14.8% and 14.2% respectively. The CR rates for M/B and M/T cases were 75.0% and 71.4% respectively. No statistical difference was observed in OS and DFS between M/B and M/T cases. CONCLUSIONS: MPAL is a rare type of acute leukemia with a high heterogeneity. The unfavorable indicators of MPAL may be factors such as abnormal karyotypes, high expression of CD34 and extramedullary infiltration. Combined regimens and more intensive therapy including allo-HSCT might contribute to improving survival.

Diagnosis of biphenotypic acute leukemia: a paradigmatic approach.

Biphenotypic acute leukemia (BAL), or acute leukemia with a single population of blasts coexpressing markers of two different lineages, is a rare clinical entity. To define BAL, a scoring system was proposed by the European Group of Immunological Markers for Leukemias (EGIL) in 1995. However, increasing evidence suggests that this system has limitations, as acknowledged by the 2008 World Health Organization (WHO) Classification of Tumors of Hematopoietic and Lymphoid Tissues. Although substantially improved in relation to the EGIL, the new WHO Classification is still not optimal for guiding the clinical management of patients with BAL. We propose a new paradigmatic approach to defining BAL based on recent clinical studies of BAL and advances in immunologic marker definition and cytogenetics, and applied our new approach to 8 cases of "BAL" among a cohort of 742 new acute leukemias in our Cancer Center. By our new criteria, 6 cases were reclassified as acute lymphoblastic leukemia (ALL), while only 2 were still classified as BAL. Our approach is also supported by analyses of the BAL cases previously reported by other institutions.

Leucemias agudas pediátricas de inmunofenotipo inusual / Childhood acute leukemias of unusual phenotype

La mayoría de las Leucemias Agudas (LA) pediátricas pueden clasificarse como Linfoblásticas (principalmente de fenotipo B o T) o Mieloblásticas dependiendo del linaje celular de los blastos, recibiendo tratamiento específico de acuerdo a esta caracterización. La inmunotipificación de las LA se basa en la evaluación de la expresión de antígenos de superficie y/o intracitoplasmáticos de diferenciación linfoide (B o T) o mieloide (My) en los blastos, lo cual permite definir la estirpe celular y clasificar la LA de acuerdo al grado de maduración. Sin embargo, existen grupos particulares poco frecuentes de LA cuya clasificación resulta dificultosa y por eso se las denomina LA de linaje ambiguo (fenotipo mixto/indiferenciadas) y LA de linaje dendrítico. Las de fenotipo mixto son aquellas en las que los blastos expresan marcadores de más de un linaje, y las indiferenciadas aquellas que no expresan antígenos específicos para ningún linaje. Diferentes convenciones se han ido desarrollando para definir y clasificar estos fenotipos inusuales, siendo la más actualizada la propuesta por la Organización Mundial de la Salud (2008). De acuerdo a estas pautas, de 1301 casos de LA diagnosticados entre abril de 1994 y abril de 2009, 28 fueron re-clasificados como LA de linaje Ambiguo, 3 como leucemia mieloide aguda minimamente diferenciadas y 3 como LA de células dendríticas. Debido a lo infrecuente de estos casos, su caracterización resulta relevante, ya que la bibliografía presenta, en general, sólo comunicaciones esporádicas de estos fenotipos particulares. Dada la importante casuística del Hospital Garrahan y el amplio seguimiento de los pacientes, el relevamiento de estos casos inusuales permite caracterizarlos desde el inmunofenotipo, la morfología/citoquímica, la citogenética/biología molecular y evaluar su presentación clínica, evolución, respuesta al tratamiento y sobrevida libre de eventos con la finalidad de colaborar con la definición de su pronóstico y eventualmente con la elaboración de protocolos de tratamiento diferenciados para estos subgrupos de LA (AU)

The majority of childhood acute leukemias (AL) can be classified as lymphoblastic (mainly phenotype B or T) or myeloblastic, depending on the cell lineage of the blasts, requiring specific treatment according to this characterization. Immunotypification of AL is based on surface and/or intracytoplasmic antigen expression with lymhoid (B or T) or myeloide (My) blast differentiation, allowing definition of cell lineage and classification of the AL according to the grade of maturation. Nevertheless, there are rare cases of AL that are difficult to classify, denominated AL of ambiguous lineage (mixed/undifferentiated lineage) and acute dendritic cell leukemia. In AL of the mixed phenotype, the blasts express markers of more than one lineage and in undifferentiated AL, the blasts lack antigen expression of any specific lineage. Different conventions have tried to define and classify these unusual phenotypes, among which the most recent proposal of the World Health Organization (2008). According to the criteria of the latter, of 1301 cases of AL diagnosed between April 1994 and April 2009, 28 were re-classified as AL of ambiguous lineage, 3 as minimally differentiated acute myeloid leukemia, and 3 as acute dendritic cell leukemia. Characterization of these cases is important, as in the literature only sporadic reports of these rare phenotypes are found. Given the large number of patients with a long follow-up of the Garrahan Hospital, a review of these unusual cases allowed characterization from the point of view of the immunophenotype, morphology/cytochemistry, cytogenetics/molecular biology and to evaluate clinical presentation, evolution, response to treatment, and event-free survival to help define the prognosis and develop protocols for the treatment of these subgroups of AL (AU)

[Biphenotypic acute leukaemia with Burkitt-like cytology]. / Leucémie aiguë biphénotypique avec aspect cytologique de type Burkitt.

Biphenotypic acute leukaemia (BAL) represents about 5% of adult acute leukaemia. Based on a previously described scoring system, the European Group for Immunologic Classification of Leukaemia (EGIL) proposed a set of diagnostic criteria for BAL. This scoring system is based on the number and degree of the specificity of several markers for myeloid or T/B lymphoid blasts. Here, we report the case of a BAL with Burkitt-like cytology, corresponding to "the acute lymphoblastic leukaemia, Burkitt type" L3 for the FAB classification. By flow cytometry, the blasts showed a positivity for B lymphoid cytoplasmic (CD79a and mu) and membrane (CD19, CD22, CD24, IgM) markers AND a positivity for the myeloid (CD13, CD33, CD65, CD15) markers.

[Cytomorphology of acute mixed leukemia]. / Citomorfologija akutnih mjesovitih leukemija.

UNLABELLED: Biphenotypic acute leukemias (AL) with blasts expressing both myeloid and lymphoid antigens are grouped with undifferentiated AL and bilineal AL in the group of AL of ambiguous lineage. Not all AL with myeloid and lymphoid antigens (ALMy+Ly) are true biphenotypic AL. According to EGIL scoring system, true biphenotypic ALMy+Ly are those with a sum of antigens 2 or more points for both myeloid and lymphoid lineage or for B and T lineage. The aim of this study was to compare cytomorphology and immunophenotype of AL to better understand the relation of certain AL morphology, immunophenotype, cytogenetics and molecular biology of biphenotypic AL. PATIENTS AND METHODS: The study included a group of 169 AL patients treated from 1985 till 1991, and a group of 102 AL patients treated from 1993 till 1996 at Zagreb University Hospital Center. Bone marrow and peripheral blood of the two groups of AL patients were analyzed according to Pappenheim (May-Grunwald-Giemsa), cytochemical and alkaline phosphatase-anti-alkaline phosphatase (APAAP) immunocytochemical staining. Flow cytometry immunophenotyping of bone marrow was also done in both patient groups. RESULTS AND DISCUSSION: In the group of 169 adult AL patients, 116 were cytomorphologically classified as acute myeloblastic leukemias (AML), 35 as acute lymphoblastic leukemias (ALL) and 18 as acute undifferentiated leukemias (ANLM). In 6 (3.4%) of 169 AL patients, blasts expressed both myeloid and lymphoid antigens. In the group of 102 AL patients there were 19 (18.6%) ALMy+Ly. In 64 patients cytomorphologically classified into AML subgroup out of 102 AL patients, there were 15 (14.7%/102; 23.4%/64) AML with lymphoid antigens (AMLLy+). In 35 patients cytomorphologically diagnosed as ALL and 3 as ANLM out of 102 AL, there were 4 (3.9%/102; 10.5%/38) ALL with myeloid antigens (ALLMy+). The incidence of mixed AL in 102 AL was more consistent with other studies, pointing to the necessity of myeloperoxidase (MPO), CD7 and TdT determination as part of standard immunophenotyping for better recognition of mixed AL. CONCLUSION: In both groups of 169 and 102 AL patients, the majority of AL cases were cytomorphologically classified as AML. In the group of 169 patients there were 5 AMLLy+ and in the group of 102 patients there were 15 AMLLy+. In one ANLM,My+ out of 169 AL and also one ANLM,My+ out of 102 AL, blasts were cytomorphologically undifferentiated; in 3 ALLMy+ of 102 AL blasts expressed lymphoid morphology. According to EGIL scoring system, among 15 AMLLy+ of 102 AL there were 4 true biphenotypic ALMy+Ly (1 M1, 2 M3, 1 M4), and in 4 ALMy+Ly with undifferentiated and lymphoid morphology there were 2 true biphenotypic AL (1 L2; 1 ANLM). In 3 ALLB+T out of 35 ALL, one was interlineal biphenotypic AL. These observations are consistent with other studies and WHO determinations indicating that the majority of true biphenotypic leukemias are associated with immature monoblastic or myeloid cytomorphology or with lymphoid or undifferentiated characteristics, but may also express any AML cytomorphology type. Thus, there is no direct correlation of leukemic cell cytomorphology and biphenotypic AL immunophenotype.

[Biphenotypic and bilineal acute leukemias]. / Bifenotipske i bilinijske akutne leukemije.

Human acute leukemias (AL) are classified as myeloid or lymphoid according to cytomorphology and the expression of leukocyte differentiation antigens/CD-markers. However, in the minority of cases leukemic cells express markers of more than one lineage, which has led to the introduction of a new subgroup of acute leukemias termed mixed or biphenotypic acute leukemias (BAL). In an effort to distinguish between BAL and those AL with aberrant expression of markers of other lineage, the European Group for the Immunological Characterization of Acute Leukemias (EGIL) has proposed a scoring system in which CD-markers are assigned a score of 0.5, 1.0 or 2.0, depending on the specificity of a particular antigen for myeloid, B- and/or T-lymphoid lineage, respectively. The new WHO classification of hematologic tumors has adopted the EGIL criteria for BAL and introduced a new group of AL termed 'AL of ambiguous lineage'. In addition to BAL in which a single cell population expresses both myeloid and lymphoid differentiation markers, this new group of leukemias also comprises cases that present with two separate blast populations (acute bilineal leukemia, aBLL). In general, BAL accounts for less than 5% of all AL cases, whereas aBLL is a rare disease constituting 1%-2% of AL cases that contains B- or T-lymphoid along with myeloid blasts. Chromosome abnormalities are frequent in both entities with a relatively high incidence of Philadelphia chromosome and rearrangements involving 11q23, especially in cases with B- and myeloid involvement. Other biological features include CD34 expression and multi-drug resistance P-glycoprotein overexpression. The prognosis of BAL and aBLL is unfavorable, with poor prognostic factors being age, high WBC and the presence of Philadelphia chromosome. Unfortunately, optimal therapy is not known, although regimens designed for acute lymphoblastic leukemia may result in a better response rate. Collaborative studies are needed for better understanding of the biology of these entities and establishment of standard therapeutic protocols.

Definition of acute biphenotypic leukemia.

BACKGROUND AND OBJECTIVE: A minority of acute leukemias have features characteristic of both the myeloid and lymphoid lineages and for this reason are designated mixed-lineage, hybrid or biphenotypic acute leukemias (BAL). There have been difficulties in establishing whether BAL represents a distinct clinico-biological entity due to a lack of objective criteria for distinguishing BAL from acute myeloid leukemias (AML) or acute lymphoblastic leukemias (ALL) with aberrant expression of a marker from another lineage. In this work we analyze diagnostic criteria for BAL. METHODS: We describe the features of 26 patients (19 adults and 7 children) with BAL diagnosed at the Royal Marsden Hospital. BAL was defined according to a scoring system devised by our group and the European Group for the Immunological Classification of Leukemia (EGIL). This system is based on the number and degree of specificity of the markers (lymphoid and myeloid) expressed by the blasts. RESULTS: According to the FAB criteria, BAL may present as "ALL" or as one of the "AML" subtypes, often M1. It is not infrequent to identify two distinct blast populations: one of small size resembling lymphoblasts and the other larger. The most common immunophenotype is coexpression of B-lymphoid and myeloid markers and less frequently, T-lymphoid and myeloid markers. Cases with a B and T lymphoid phenotype or with trilineage differentiation are rare. BAL has a high incidence of clonal chromosomal abnormalities, the most common being the t(9;22) (q34;q11) (Ph chromosome) and structural abnormalities involving 11q23. Data are emerging that BAL has a negative prognosis in both children and adults and this may be related to the underlying chromosome abnormalities. INTERPRETATION AND CONCLUSIONS: In summary, BAL is an uncommon type of leukemia which probably arises from a multipotent progenitor cell and carries a poor prognosis. Although there are no uniform criteria about whether to treat these patients as ALL or AML, it is likely that an intensive approach with high-dose therapy followed by bone marrow transplantation will be required to eradicate the disease permanently.

Possibilities and limitations of cytochemical methods in diagnosis of acute leukemia.

Accurate identification and classification of leukemic blast cells is a very important prerequisite of the precise diagnosis of acute leukemia and has a great impact on therapy and prognosis. The purpose of this review is to consider, in the broad sense of the word, the present possibilities and limitations of enzyme cytochemistry and to emphasize how cytochemistry may contribute, on integration with the other methods of study, to the final classification and differential diagnosis of acute leukemia, a highly variable hematological disorder. In this review, the role of conventional enzyme cytochemistry, either dominant or subsidiary, in the discrimination of acute leukemia subtypes is discussed. The survey confirms the absolute necessity of immunologic marker analysis in the accurate diagnosis of acute lymphoblastic leukemia, undifferentiated or minimally differentiated leukemia and mixed-lineage leukemia because in these cases, the cytochemical evaluation provides insufficiently relevant information regarding blast cell origin, specificity of leukemia subtypes and the discrete stages of leukemic cell maturation. On the other hand, cytochemical investigation is appreciated to be dominant over immunophenotyping in characterizing acute myeloid leukemia, because of the lack of specificity of the majority of immunological markers against myeloid antigens and, because of the availability of standardized and sufficiently specific cytochemical reactions. The cytogenetic, molecular biological and electron microscopic studies mentioned in this review supplement the important information for correct differential diagnosis of acute leukemia. The prognostic impact of enzyme cytochemistry in correlation to other methods is evaluated.