The presence of a novel, minute chromosome m the cells of patients with chrome myeloid leukemia (CML) was first described m 1960 by Nowell and Hungerford (1 ). The Philadelphia (Ph) chromosome, as it became known, was shown subsequently by banding techniques to result from a recrprocal translocanon between the long arms of chromosomes 9 and 22 t(9;22) (q34;qll) (2 ). Molecular studies demonstrated that the translocatton disrupted the normal ABL and BCR genes on chromosomes 9 and 22, respectively, grvmg rise to a chimeric BCR-ABL gene encoding a fusion protein with transforming ability (3 ). The reciprocal ABL-BCR product is also transcriptionally active in the majority of cases (4 , 5 ).
