The primary structure of normal abl protein was determined by sequencing the coding region of its cDNA. abl contains two alternative 5' exons spliced to a common set of 3' exons to yield the two major abl RNA transcripts. These transcripts initiate in different promoter regions and give rise to proteins that vary in their N-termini. In the human cell line K562, abl is translocated from chromosome 9 to within the bcr gene on chromosome 22. Within the fused bcr-abl gene, abl exon II alternatively splices to two adjacent bcr exons. This phenomenon is seen in many patients with chronic myeloid leukemia.