LAUSR

Dear Editors, Acute myeloid leukemia (AML) with inv(16)(p13q22)/t(16;16) (p13;q22) has distinct clinicopathologic features, including myelomonocytic leukemic cells, abnormal bone marrow eosinophils, and a relatively favorable prognosis.1,2 Most cases are classified as AML M4 with abnormal eosinophils (M4Eo) in the FrenchAmericanBritish (FAB) classification. The inv(16)/t(16;16) leads to a fusion gene between the corebinding factor beta subunit (CBFB) at 16q22 and myosin heavy chain 11 (MYH11) at 16p13. More than 10 types (types A– K) of CBFB/MYH11 fusion transcripts have been characterized.3 Furthermore, in addition to inversions and translocations, a few cases of insertions leading to the CBFB/MYH11 fusion gene, namely, insertion of CBFB into MYH11 or insertion of MYH11 into CBFB, have been reported.410 We describe here a rare case of AML with a normal karyotype and type D CBFB/MYH11 fusion transcript. Fluorescence in situ hybridization (FISH) revealed an atypical signal pattern by cryptic insertion. A 53yearold man was admitted due to fever and leukocytosis. He had no history of chemotherapy or radiotherapy for malignancies. Peripheral blood showed hemoglobin 52 g/L, platelets 21 × 109/L, and white blood cells 41.0 × 109/L, with 5% neutrophils, 16% lymphocytes, 34% monocytes, and 45% blasts. Bone marrow was hypercellular, with 14.8% monocytes, 5.6% eosinophils, and 74.8% blasts staining positive for myeloperoxidase (Figure 1A,B). Flow cytometric analysis revealed blasts positive for CD13 (77.0%), CD33 (81.5%), CD34 (29.7%), and HLADR (85.8%). Gbanding analysis of bone marrow cells showed 46,XY[20] (Figure 2A). Reverse transcription polymerase chain reaction (RTPCR) screening detected a CBFB/MYH11 fusion transcript, whereas other fusions, including BCR/ABL1, RUNX1/RUNX1T1, PML/RARA, and DEK/NUP214, were negative. Furthermore, NPM1, FLT3D835, and KIT (exons 7 to 17) mutations and FLT3internal tandem duplication were also negative. To perform RTPCR for CBFB/MYH11, we created a forward primer, CBFBF (CBFB