Trisomy 4 in a case of acute biphenotypic leukemia.

Trisomy 4 was the sole chromosome anomaly in a 5-year-old girl with acute leukemia. Morphologically, there appeared to be distinct myeloid and lymphoid blast cells on presentation. Immunophenotyping, however, showed extensive overlap of myeloid and lymphoid markers, confirming the leukemia to be biphenotypic rather than true "bilineal." She attained remission only after lymphoid-cell-specific induction was added to the initial "myeloid type" induction. She relapsed 4 years later with morphologically acute lymphocytic leukemia (French-American-British L2 type) despite still retaining the original immunophenotypic characteristics. She was successfully reinduced and subsequently received an autologous bone marrow transplant. Second relapse, morphologically and immunophenotypically similar to the first, occurred 5 months after transplant.