The BCR/ABL fusion gene is pathognomonic for chronic myelogenous leukaemia (CML). We have previously reported alternative splicing of BCR/ABL, as indicated by the detection of both p190- and p210-encoding transcripts, in about 60% of CML patient samples. These exon-skipping events involved the joining of ABL exon 2 to variable upstream BCR exons. Similarly, ABL exon 2 is alternatively spliced to either of two upstream ABL exons (1a or 1b) in c-ABL. We have constructed BCR and BCR/ABL minigenes to study this phenomenon in more detail. These constructs were transfected into various cell types and splicing was assessed by reverse transcriptase PCR. Whereas the basic BCR minigene expressed exon-inclusive transcripts only, insertion of genomic DNA spanning ABL exon 2 induced exon-skipping but only when expressed in the CML cell lines K562 and EM3. In this study we localized the required sequence element to ABL exon 2 itself. These results mimic the splicing phenotype displayed by most CML patients. We propose a model where a trans-factor present in some CML cells interacts with ABL exon 2 pre-mRNA to promote skipping of upstream BCR exons.