A Philadelphia-negative chronic myeloid leukemia with a BCR/ABL fusion gene on chromosome 9.

A 40-year-old man had chronic myeloid leukemia (CML) and an apparently normal karyotype. Fluorescence in situ hybridization with a BCR/ABL1-S probe, which is formatted to display a BCR/ABL fusion signal on chromosome 22, gave a positive fusion signal on a chromosome 9. Therefore this patient has a BCR/ABL fusion gene on chromosome 9. The BCR/ABL1-D probe, formatted to display a fluorescent signal for both the reciprocal products of a 9/22 rearrangement, gave a positive fusion signal on the derivatives 9 and 22. These findings favor either a cryptic reciprocal exchange between BCR and ABL loci or the reversal of a Philadelphia translocation. An insertion of BCR next to ABL is ruled out. The reverse-transcriptase polymerase chain reaction provided molecular evidence that a typical CML chimeric product resulting from a fusion of BCR exon 2 with C-ABL exon II, a2b2, is present.