[Acute lymphoblastic leukemia with "masked" Philadelphia chromosome].

A 76-year-old woman with leukocytosis and thrombocytopenia was admitted to our hospital. A bone marrow examination showed a composition of 82.0% blasts, i.e., positive for TdT, CD10, CD19, CD34, and HLA-DR and negative for cyCD3, CD13, CD33, MPO, and cyµ. The reverse transcription-polymerase chain reaction analysis revealed a minor BCR-ABL1 fusion gene, leading to a diagnosis of acute lymphocytic leukemia (ALL) with a BCR-ABL1 fusion gene. G-band assay was negative for Philadelphia (Ph) chromosome and also revealed add (21) (q22. 1) and del (20) (q11. 2q13.3). Fluorescence in situ hybridization (FISH) assaying revealed a positive BCR-ABL1 fusion signal. Thus, this patient was diagnosed as Ph chromosome-negative and BCR-ABL1-positive fusion gene ALL, which suggested the presence of ALL with the "masked" Ph chromosome found in approximately 1% of chronic myeloid leukemia. Therefore, the FISH analysis may complement cytogenetic analysis when cytogenetic and molecular genetic findings are contradictory in ALL.