Derivative (6)t(1;6)(q22;p21) revealed in bone marrow cells by FISH 9 months before diagnosis of acute T-lymphoblastic leukemia.

A chromosomal clone with unbalanced translocation resulting in partial trisomy of segment 1q22-qter and partial monosomy of segment 6p21-pter was revealed by fluorescence in situ hybridization (FISH) using a panel of different whole chromosome painting probes. The pathologic clone appeared after sequential chemotherapy treatment for AML-M5 when the patient was in complete remission before development of T-ALL. However, this clone was present during the whole period of treatment for T-ALL. The clone remained the only chromosomal aberration found. Breakpoints were detected more easily and more precisely with the use of the FISH technique than with G-banding only.