Chromosomal translocation t(9; 22)(q34; q11), found in 95% of patients with chronic myeloid leukemia(CML) and 30% of adult patients with acute lymphoblastic leukemia (ALL) generates a chimeric gene, BCR/ABL. There are three kinds BCR/ABL fusion transcripts of p210BCR-ABL found in CML and ALL, p190BCR-ABL mainly in ALL, and p230BCR-ABL in CML, either of which depends on the location of the breakpoints within the BCR gene. For the detection of t(9; 22) or BCR/ABL, karyotype analysis, Southern blot hybridization of the BCR gene, fluorescence in situ hybridization, and reverse transcription-polymerase chain reaction(RT-PCR) have been used. Especially, recent advance in RT-PCR methods have allowed refined quantitative detection of the BCR/ABL transcripts, which are useful for monitoring response status and detecting minimal residual disease.