Rapid diagnosis of acute promyelocytic leukemia by analyzing the immunocytochemical pattern of the PML protein with the monoclonal antibody PG-M3

Promyelocytic Leukemia Antigen Expression: a Histological Marker for Primary Biliary Cholangitis Diagnosis?

Background and Objectives

Distinguishing primary biliary cholangitis (PBC) from other cholestatic diseases at the histological level could be assisted by new methods, such as immunohistochemical staining of specific antigens.

Methods

We evaluated whether the detection of promyelocytic leukemia protein (PML) can serve as a specific and sensitive marker for PBC diagnosis. Liver biopsies from 26 PBC patients, 20 primary sclerosing cholangitis (PSC), 37 viral hepatitis, 11 non-alcoholic steatohepatitis (NASH) and 5 normal patients were investigated after immunostaining with the anti-PML monoclonal PG-M3, IgG1 antibody.

Results

Immunoreactivity in bile ducts was expressed by the PML-score (quotient of positive ducts to the total number of portal tracts multiplied by 2). PML-score was higher in PBC as compared to controls (P < 0.001). Using a cutoff of 0.18, PML-score proved highly sensitive (84.6%) and specific (89.7%) for confirming PBC as compared to only 5% of PSC, 9.1% of NASH and 13.5% of viral hepatitis patients (P < 0.001). Irrespective of the underlying disease, patients with PML-score > 0.18 were older (P = 0.007), more often females (P < 0.001) with higher ALP (P < 0.001), γ-GT (P = 0.001) and IgM (P < 0.001) compared to the patients with PML-score < 0.18.

Conclusions

We postulate that a simple PML immunohistochemical test could be sufficient for histopathological discrimination of PBC in problematic cases of undefined cholestatic disorders, including small-duct PSC and AMA-negative PBC cases.

Early treatment of acute promyelocytic leukaemia is accurately guided by the PML protein localisation pattern: real-life experience from a tertiary New Zealand centre

Current guidelines recommend that a rapid test be used to assist diagnosis of acute promyelocytic leukaemia (APL), but the choice of an assay is discretionary. PML immunofluorescence (PML IF) identifies the microparticulate pattern of the PML protein localisation, highly specific for APL. The aim of this study was to evaluate clinical utility of PML IF in a real-life setting based on a retrospective records review for all patients who had PML IF performed in our centre between 2000 and 2017. Final analysis included 151 patients, 70 of whom had APL. PML IF was reported on average 3 days faster than cytogenetics. Compared with genetic results, PML IF showed sensitivity of 96% and specificity of 100%. PML IF accurately predicted APL in four APL cases with cryptic karyotype/FISH and excluded APL in 98% cases tested based on the suspicious immunophenotype alone, 21/28 of whom had mutated NPM1. Results of PML IF influenced decision to start ATRA in 25 (36%) APL patients and led to its termination in six non-APL patients. In conclusion, PML IF is a fast and reliable test that facilitates accurate treatment decisions when APL is suspected. This performance of PML IF remains hard to match in a real-life setting.

Immunostaining for rapid diagnosis of acute promyelocytic leukemia with the tetramethylrhodamine-5-isothiocyanate-conjugated anti-promyelocytic leukemia monoclonal antibody PG-M3

CONTEXT

Anti-promyelocytic leukemia (PML) immunofluorescence staining is a known diagnostic tool for rapid diagnosis of acute promyelocytic leukemia (APL).

OBJECTIVE

We describe our methods using the recently developed, commercially available, tetramethylrhodamine-5-isothiocyanate-labeled PG-M3 anti-PML antibody for APL testing.

DESIGN

Immunofluorescence staining with the tetramethylrhodamine-5-isothiocyanate-labeled PG-M3 antibody was used to detect PML-RARA in bone marrow aspirate and/or peripheral blood smears from 30 patients with acute leukemia. The results were compared with those of concurrent testing with our in-house polyclonal anti-PML antibody and with established tests.

RESULTS

All APL cases showed a positive (fine/microgranular) immunofluorescence staining pattern, whereas non-APL cases showed a negative (chunky/macrogranular) pattern. These results, which were available within 2 hours, were validated by testing with the polyclonal anti-PML antibody and with established cytogenetic and molecular testing methods.

CONCLUSIONS

We validated the utility of the tetramethylrhodamine-5-isothiocyanate-labeled anti-PML antibody PG-M3 for the diagnosis of APL. Our results indicate that immunofluorescence staining with this antibody is a rapid and reliable method for the diagnosis of APL.

Personalized medicine and pharmacogenetic biomarkers: progress in molecular oncology testing

In the field of oncology, clinical molecular diagnostics and biomarker discoveries are constantly advancing as the intricate molecular mechanisms that transform a normal cell into an aberrant state in concert with the dysregulation of alternative complementary pathways are increasingly understood. Progress in biomarker technology, coupled with the companion clinical diagnostic laboratory tests, continue to advance this field, where individualized and customized treatment appropriate for each individual patient define the standard of care. Here, we discuss the current commonly used predictive pharmacogenetic biomarkers in clinical oncology molecular testing: BRAF V600E for vemurafenib in melanoma; EML4-ALK for crizotinib and EGFR for erlotinib and gefitinib in non-small-cell lung cancer; KRAS against the use of cetuximab and panitumumab in colorectal cancer; ERBB2 (HER2/neu) for trastuzumab in breast cancer; BCR-ABL for tyrosine kinase inhibitors in chronic myeloid leukemia; and PML/RARα for all-trans-retinoic acid and arsenic trioxide treatment for acute promyelocytic leukemia.

Flow cytometric immunobead assay for fast and easy detection of PML-RARA fusion proteins for the diagnosis of acute promyelocytic leukemia

The PML–RARA fusion protein is found in approximately 97% of patients with acute promyelocytic leukemia (APL). APL can be associated with life-threatening bleeding complications when undiagnosed and not treated expeditiously. The PML–RARA fusion protein arrests maturation of myeloid cells at the promyelocytic stage, leading to the accumulation of neoplastic promyelocytes. Complete remission can be obtained by treatment with all-trans-retinoic acid (ATRA) in combination with chemotherapy. Diagnosis of APL is based on the detection of t(15;17) by karyotyping, fluorescence in situ hybridization or PCR. These techniques are laborious and demand specialized laboratories. We developed a fast (performed within 4–5 h) and sensitive (detection of at least 10% malignant cells in normal background) flow cytometric immunobead assay for the detection of PML–RARA fusion proteins in cell lysates using a bead-bound anti-RARA capture antibody and a phycoerythrin-conjugated anti-PML detection antibody. Testing of 163 newly diagnosed patients (including 46 APL cases) with the PML–RARA immunobead assay showed full concordance with the PML–RARA PCR results. As the applied antibodies recognize outer domains of the fusion protein, the assay appeared to work independently of the PML gene break point region. Importantly, the assay can be used in parallel with routine immunophenotyping for fast and easy diagnosis of APL.

Modern Approaches to Treating Acute Promyelocytic Leukemia

The advent of all-trans-retinoic acid (ATRA) and its combination with anthracycline-containing chemotherapy have contributed in the past 2 decades to optimize the antileukemic efficacy in acute promyelocytic leukemia (APL), leading to complete remission rates greater than 90%, virtual absence of resistance, and cure rates of nearly 80%. Recently reported studies from large cooperative trials have also shown that more rational delivery of treatment and improved outcomes may derive from the use of risk-adapted protocols. In particular, patients at higher risk of relapse (ie, those presenting with WBC > 10 × 109/L) seem to benefit from treatments that include cytarabine in the ATRA-plus-chemotherapy scheme, whereas patients with standard-risk disease can be successfully managed with less-intensive regimens that contain ATRA and anthracycline-based chemotherapy. After the outstanding results with arsenic trioxide (ATO) in the treatment of APL relapse, several experimental trials have been designed to explore the role of ATO in front-line therapy with the aim not only of minimizing the use of chemotherapy but also to reinforce standard ATRA-plus-chemotherapy regimens and additionally improve therapeutic efficacy. In this review article, we discuss most recent advances in the treatment of patients with newly diagnosed and relapsed APL.

Immunohistochemical surrogates for genetic alterations of CCDN1, PML, ALK, and NPM1 genes in lymphomas and acute myeloid leukemia

The World Health Organization (WHO) classification of lympho-hemopoietic neoplasms is increasingly based on genetic criteria. Detection of tumor-associated primary genetic lesions is usually performed using the polymerase chain reaction (PCR) and/or fluorescence in-situ hybridization (FISH). This review focuses on alternative techniques for detecting genetic lesions in biopsy samples. Immunohistochemical surrogates for the detection of genetic alterations involving the CCND1, PML, anaplastic lymphoma kinase (ALK) and nucleophosmin (NPM1) genes are presented as examples for this approach. Because of their high specificity, rapidity, low costs and ease of performance, these assays have the potential for being extensively applied in developing countries. In some instances (e.g. detection of ALK protein) immunohistochemistry has fully replaced molecular studies for routine diagnosis in paraffin-embedded specimens. Genome wide based discovery of new tumor-associated genetic lesions that are suitable for antibody targeting promises to further expand the application of immunohistochemistry for the molecular classification of hematological neoplasms.

Evaluation of PG-M3 antibody in the diagnosis of acute promyelocytic leukaemia

BACKGROUND & OBJECTIVES

Acute promyelocytic leukaemia (APL) is a distinct subtype of acute myeloid leukaemia (AML) characterized by a reciprocal translocation, t(15;17) and a high incidence of life-threatening coagulopathy. APL diagnosis is considered a medical emergency. As reverse transcription-polymerase chain reaction (RT-PCR) for PML-RAR fusion oncoprotein is time consuming, there is a need for a rapid and accurate diagnostic test for APL. This study evaluates the role of PG-M3 monoclonal antibody using immunofluorescence (IF) in the early diagnosis of APL.

MATERIALS AND METHODS

Thirty-six new untreated APL cases diagnosed with RT-PCR for PML-RAR as the gold standard and 38 non-APL controls (28 non-APL AMLs and 10 non-leukaemic samples) were evaluated by routine morphology and cytochemistry, RT-PCR and IF using PG-M3 monoclonal antibody.

RESULTS

Using IF, 34 of 36 (94·4%) APL cases showed a microgranular pattern suggestive of APL and two cases (5·6%) showed a speckled pattern typical of wild-type PML protein (False negative). By comparison, two of 28 (7·1%) non-APL AMLs showed microgranular pattern (false positive). Hence, IF as a diagnostic test for APL resulted in a sensitivity of 94·4%, specificity of 92·9% and positive and negative predictive values of 94·4% and 92·9% respectively. All 10 non-leukaemic samples showed a speckled pattern.

CONCLUSIONS

IF using PG-M3 antibodies can be used as a rapid (takes 2 h), cheap, sensitive and specific method to identify APL. It can be a useful adjunct for diagnosis of APL especially if facilities for RT-PCR are not available, particularly in resource-limited settings.

Rapid and reliable confirmation of acute promyelocytic leukemia by immunofluorescence staining with an antipromyelocytic leukemia antibody: the M. D. Anderson Cancer Center experience of 349 patients

BACKGROUND

The authors evaluated the utility of immunofluorescence staining with an antipromyelocytic leukemia (anti-PML) antibody for patients with a suspected diagnosis of new or relapsed acute promyelocytic leukemia (APL) and correlated the findings with the results of other established diagnostic modalities.

METHODS

Bone marrow (BM) and/or peripheral blood (PB) smears from 349 patients in whom the diagnosis of APL was considered were assessed with the anti-PML antibody using immunofluorescence. The study group included 199 patients with confirmed APL and 150 with other conditions. The results of conventional cytogenetics, reverse transcription polymerase chain reaction (RT-PCR), and fluorescence in situ hybridization (FISH) performed on these patients were correlated with the PML results.

RESULTS

Among patients with confirmed APL, anti-PML antibody was positive in 182 of 184 BM and 32 of 33 PB smears. Conventional cytogenetics demonstrated t(15;17)(q22;q12) in 166 of 182 (91%) patients; 10 had a normal karyotype, 4 had insufficient mitoses to grow in culture, 1 was inconclusive, and 1 was 48, XX, +8, +8. Anti-PML staining was positive in 9 of 10 with a normal karyotype and in all 4 cases with insufficient mitoses. RT-PCR and FISH were positive for PML-retinoic acid receptor-alpha in 169 of 172 (98%) and 90 of 94 (96%) cases, respectively. Among the patients without APL, 148 of 150 (98.6%) were negative with anti-PML antibody. The sensitivity and specificity of the test were 98.9% and 98.7%, respectively.

CONCLUSIONS

PML immunofluorescence staining is a rapid (<4 hours turnaround time) and reliable frontline diagnostic approach that can facilitate initiation of targeted therapy, particularly in clinical settings where cytogenetic and molecular testing are not readily available.

Management of acute promyelocytic leukemia: recommendations from an expert panel on behalf of the European LeukemiaNet

The introduction of all-trans retinoic acid (ATRA) and, more recently, arsenic trioxide (ATO) into the therapy of acute promyelocytic leukemia (APL) has revolutionized the management and outcome of this disease. Several treatment strategies using these agents, usually in combination with chemotherapy, but also without or with minimal use of cytotoxic agents, have provided excellent therapeutic results. Cure of APL patients, however, is also dependent on peculiar aspects related to the management and supportive measures that are crucial to counteract life-threatening complications associated with the disease biology and molecularly targeted treatment. The European LeukemiaNet recently appointed an international panel of experts to develop evidence- and expert opinion-based guidelines on the diagnosis and management of APL. Together with providing current indications on genetic diagnosis, modern risk-adapted front-line therapy and salvage treatment, the review contains specific recommendations for the identification and management of most important complications such as the bleeding disorder, APL differentiation syndrome, QT prolongation and other ATRA- and ATO-related toxicities, as well as for molecular assessment of response to treatment. Finally, the approach to special situations is also discussed, including management of APL in children, elderly patients, and pregnant women.

Front line clinical trials and minimal residual disease monitoring in acute promyelocytic leukemia

In spite of the very high cure rate (70%-80%) achieved in APL with combinatorial all-trans retinoic acid (ATRA) and anthracycline-based chemotherapy regimens, a number of issues are still open for investigation in front-line therapy of this disease. These include, among others, improvements in early death rate, the role of arsenic trioxide (ATO) and maintenance treatment, and, finally, optimization of molecular monitoring to better identify patients at increased risk of relapse. The current consensus on the most appropriate induction therapy consists of the concomitant administration of ATRA and anthracycline-based chemotherapy. Although the antileukemic benefit provided by the addition of ATRA to consolidation therapy has not been demonstrated in randomized studies, historical comparisons of consecutive studies carried out by Spanish and Italian cooperative groups suggest that the combination of ATRA and chemotherapy for consolidation may also contribute to improving therapeutic results. While a variety of distinct treatments are being investigated for front-line therapy, most experts agree that a risk-adapted therapy represents the optimal approach, through the use of more intensive therapy in patients with initial hyperleukocytosis. Longitudinal RT-PCR of PML/RARalpha allows sensitive assessment of response to treatment and minimal residual disease (MRD) monitoring in APL. Achievement of negative PCR status or molecular remission at the end of consolidation is now universally accepted and recommended as a therapeutic objective in this disease. On the other hand, persistence of, or conversion to, PCR positive in the marrow during follow-up is associated with impending relapse. Preliminary studies on therapy of molecular relapse indicate a survival advantage as compared to administering salvage treatment at time of hematologic relapse. The more accurate and reproducible real-time PCR method to detect at quantitative levels the PML/RARalpha hybrid will likely provide better inter-laboratory standardization and trial results comparison in the near future.

Immunohistochemical analysis of CBFbeta-SMMHC protein reveals a unique nuclear localization in acute myeloid leukemia with inv(16)(p13q22)

The inv(16)(p13q22) or, less commonly the t(16;16)(p13;q22), is characteristic of acute myeloid leukemia (AML) with abnormal bone marrow eosinophils, also known as AML-M4Eo. This abnormality creates a fusion gene, 5' core binding factor beta (CBF-beta) gene and the 3' MYH11 gene, the latter encoding smooth muscle myosin heavy chain gene (SMMHC). Detection of this abnormality is important for diagnosis and is most commonly done by cytogenetics or molecular methods. In this study, we determined the utility of immunohistochemical and immunofluorescence methods using a rabbit polyclonal antibody (AH107) against the C-terminus of the CBFbeta-SMMHC chimeric protein for diagnosis of AML-M4Eo. Thirty-nine AML-M4Eo cases and 55 cases of other types of AML were evaluated. Immunohistochemical analysis of routinely processed paraffin-embedded bone marrow sections showed that CBFbeta-SMMHC staining is predominantly nuclear in all cases of AML-M4Eo and is not nuclear in other AML types. Four cases of AML-M4Eo double-stained for CBFbeta-SMMHC and CD34 showed the fusion protein in CD34-positive blasts. Indirect immunofluorescence analysis of fresh bone marrow aspirate smears showed that AML-M4Eo blasts have a distinct nuclear microgranular or fine-speckled pattern of staining, with or without faint cytoplasmic staining. By contrast, other types of AML and normal bone marrow specimens were either negative or had a nonspecific pattern of staining. In summary, immunostaining for CBFbeta-SMMHC using either immunohistochemical or immunofluorescense analysis as described here reveals a distinctive pattern of staining for AML-M4Eo. This approach is a specific, reliable, and convenient alternative to cytogenetic and molecular methods for the diagnosis of AML-M4Eo and may be particularly helpful in cases with indeterminate histologic features or in cases in which cytogenetic and molecular studies are either uninformative or not available.

Practice points, consensus, and controversial issues in the management of patients with newly diagnosed acute promyelocytic leukemia

Recent reviews on acute promyelocytic leukemia (APL) treatment have focused on comparing therapeutic approaches, including all-trans retinoic acid (ATRA) and chemotherapy, and do not address several other aspects of APL management that are relevant to the outcome in individual patients. These aspects include appropriate diagnostic tools and strategies, supportive care, recognition and treatment of life-threatening complications, evaluation of response, and, finally, management of the disease in special conditions such as older patients and pregnant women. In addition to reviewing current consensus and controversies of ATRA and chemotherapy treatment, this article addresses the above issues of APL management with special emphasis on aspects that distinguish APL from other acute myelogenous leukemias.

Tricks of the trade for the appropriate management of newly diagnosed acute promyelocytic leukemia

Most reviews on the state-of-the-art treatment in acute promyelocytic leukemia (APL) have focused mainly on the comparison of therapeutic approaches, including all-trans retinoic acid (ATRA) and chemotherapy. However, outcome of individual patients also depends on appropriate knowledge of several aspects related to APL management that are less appreciated and/or are underestimated in the literature. These aspects include appropriate diagnostic strategy, use of supportive care, early recognition and treatment of life-threatening complications typically associated with APL and its specific treatment, tools and timing for adequate evaluation of response, and, finally, management of the disease in special conditions such as older patients and pregnant women. Besides reviewing current consensus and controversies on the use of ATRA and chemotherapy in the distinct treatment phases (eg, induction, consolidation, maintenance), this article addresses the aforementioned issues on APL management (“tricks of the trade”) with special emphasis on several peculiar aspects that distinguish APL from other acute myeloid leukemias.

Immunofluorescent analysis with the anti-PML monoclonal antibody PG-M3 for rapid and accurate genetic diagnosis of acute promyelocytic leukemia

Genetic diagnosis is currently considered the most reliable method to accurately identify patients with acute promyelocytic leukemia (APL) requiring tailored therapy including all- trans retinoic acid (ATRA). We investigated the clinical effectiveness of immunofluorescence techniques with the anti-PML monoclonal antibody PG-M3 for rapid and accurate diagnosis of APL. PML immunofluorescence staining was analyzed in 164 patients with acute myeloblastic leukemia (AML), including APL (110 patients) and non-APL subtypes (54 patients). All 54 patients with an AML phenotype, in whom tests for t(15;17) or its fusion gene PML/ RARalpha were negative, showed a speckled (macrogranular) nuclear pattern. Of the 110 genetically diagnosed APL patients, 108 showed a microgranular pattern that confirmed PG-M3 positivity. The remaining two patients were not evaluable for PG-M3 reactivity because of scarcity of cells. No patient with APL showed a normal pattern. The high sensitivity and specificity of immunolabeling using PG-M3 monoclonal antibody show that it is a highly efficient and reliable tool to identify PML/ RARalpha-positive patients with APL and that it should be standardized as a first-line diagnostic procedure. In addition, it is technically simple, fast, and cheap, only requiring small tissue samples and non-sophisticated equipment.

Diagnostic value of detecting fusion proteins derived from chromosome translocations in acute leukaemia

Clonal chromosomal abnormalities such as balanced translocations are characteristic features of several human leukaemias and have long been detected by conventional cytogenetics on banded metaphases. The advent of molecular biology techniques, advanced karyotyping and immunohistochemistry methods has not only allowed identification of gene involvement at altered chromosome sites and better knowledge of leukaemia pathogenesis, but also contributed important improvements in diagnosis of these heterogeneous diseases. Such novel diagnostic strategies are nowadays being increasingly used to improve leukaemia classification, and in several instances, they help to establish the most appropriate therapeutic strategy in individual patients. Moreover, at least two leukaemia-associated fusion proteins derived from chromosome translocation are specifically targeted by therapeutic approaches which result in significantly increased anti-leukaemia efficacy and reduced toxicity. In this chapter, we highlight the importance of identifying these genetic lesions at diagnosis in acute leukaemia. Further, we discuss briefly the clinical utility of detecting these alterations for prognostic assessment and evaluation of response to treatment.

Leukemias resembling acute promyelocytic leukemia, microgranular variant

Acute promyelocytic leukemia (APL) should be distinguished from other subtypes of acute myeloid leukemia (AML) because of the increased risk of disseminated intravascular coagulation (DIC) and its response to arsenic compounds and retinoids. Some cases of AML seem morphologically similar to the microgranular variant of APL (French-American-British [FAB] AML-M3v) but lack the t(15;17). We evaluated 8 cases of APL-like leukemias for subtle morphologic, cytochemical, immunophenotypic, and cytogenetic differences compared with 5 cases of promyelocytic leukemia/retinoic receptor alpha (PML/RARalpha)-positive APL (FAB AML-M3v). We also evaluated both groups for the presence of DIC. No differences among the groups were noted in blast size, chromatin pattern, nuclear morphologic features, intensity of myeloperoxidase staining, or presence of Auer rods. Immunophenotypes were similar; both types of cases lacked CD34 and HLA-DR and were CD13+ and CD33+. Two cases of APL-like leukemias also were CD56+. DIC was present in 2 patients with M3v. Our study shows that there are no definitive morphologic, cytochemical, or immunophenotypic findings that can distinguish these cases from PML/RARalpha-positive APL.

Isolated myelosarcoma in children--update and review

Primary myelosarcomas, also called leukemia cutis, granulocytic sarcomas or chloromas, are rare extramedullary manifestations of acute myeloid leukemia (AML) which precede bone marrow involvement. Skin infiltration was the most frequent localization associated with a myelomonocytic differentiation. Although first remission was achieved by most children, risk of relapse seemed to be increased. This might be caused by the specific biology of myelosarcomas, but also may be the result of delayed or reduced treatment. During the AML-BFM-studies 87/93/98 (11/1987-7/2000) 37 children with isolated myelosarcomas were diagnosed. Eighteen of the 37 patients survived with a 5-year overall survival estimation of 0.54+/-0.09 compared to 0.59+/-0.02; p(log rank) = 0.94. However, reduced or delayed treatment in 17 children led to an increased relapse rate of 71% compared to 35% in children treated soon after diagnosis. The 5-year overall survival in these patients was 0.41+/-0.11. According to our experience and review of the literature, an early diagnostic workup is needed in children with unusual skin lesions or tumors, considering myelosarcoma as primary manifestation of AML. Intensive AML-specific chemotherapy is generally recommended soon after diagnosis.

Proteins encoded by genes involved in chromosomal alterations in lymphoma and leukemia: clinical value of their detection by immunocytochemistry

Acquired chromosomal anomalies (most commonly translocations) in lymphoma and leukemia usually result in either activation of a quiescent gene (by means of immunoglobulin or T-cell-receptor promotors) and expression of an intact protein product, or creation of a fusion gene encoding a chimeric protein. This review summarizes current immunocytochemical studies of these 2 categories of oncogenic protein, with emphasis on the clinical relevance of their detection in diagnostic samples. Among the quiescent genes activated by rearrangement, expression of cyclin D1 (due to rearrangement of the CCND1 [BCL-1] gene) is a near-specific marker of t(11;14) in mantle cell lymphoma; BCL-2 expression distinguishes follicular lymphoma cells from their nonneoplastic counterparts in reactive germinal centers and appears to be an independent prognostic marker in diffuse large cell lymphoma; and TAL-1 (SCL) expression identifies T-cell acute lymphoblastic neoplasms in which this gene is activated. The protein products of other genes activated by chromosomal rearrangement have a role as markers of either lineage (eg, PAX-5 [B-cell-specific activator protein] for B cells, including B-lymphoblastic neoplasms), or maturation stage (eg, BCL-6 for germinal-center and activated B cells and MUM-1/IRF4 for plasma cells). Currently, no hybrid protein encoded by fusion genes is reliably detectable by antibodies recognizing unique junctional epitopes (ie, epitopes absent from the wild-type constituent proteins). Nevertheless, staining for promyelocytic leukemia (PML) protein will detect acute PML with t(15;17) because the microspeckled nuclear labeling pattern for PML-RARalpha is highly distinctive. Similarly, antibodies to the anaplastic lymphoma kinase (ALK) tyrosine kinase are valuable (because wild-type ALK is not found in normal lymphoid tissue) in detecting neoplasms (CD30-positive large T-cell lymphomas) with t(2;5) or its variants. Thus, immunocytochemical detection of the products of many rearranged genes in lymphoma and leukemia can be clinically informative and provide information on cellular and subcellular protein expression that cannot be inferred from studies based on messenger RNA.