Schematic representation of the types of leukemia-associated BCR-ABL transcripts that can be amplified in the two PCR protocols. Primers for first-step and second (nested)-step amplifications are indicated as arrows over the exon regions corresponding to their sequences. The upper diagram shows the BCR gene structure according to Chissoe et al29 and the major and minor breakpoint cluster regions (M-bcr and m-bcr, respectively). Because of the position of the BCR primers, the p210 PCR assay can only detect transcripts derived from breaks in M-bcr, whereas the p190 PCR assay is able to amplify fusion transcripts arising from breakpoints in either m-bcr or M-bcr (ie, from alternative splicing of the primary M-bcr derived message).