Human chronic myelogenous leukemia (CML) is a clonal hematologic disorder. CML is characterized by the t(9:22) chromosome translocation which results in translocation of the oncogene abl from chromosome 9 into the breakpoint cluster region (bcr) gene on chromosome 22. We cloned and characterized the cDNA of the normal bcr gene. The bcr gene codes for a protein of 1271 amino acids. The open reading frame is preceded by a region high in GC. At the 5' of this region we identified several GC motifs which are probably involved in the initiation of bcr transcription. bcr transcripts of 7.0 and 4.5 kb are expressed in all cell types examined. These transcripts share all cDNA sequences analysed, including the 5' untranslated region. The latter as well as 902 or 927 amino acids are included within the CML-specific bcr-abl mRNA transcribed from the chimeric bcr-abl gene on chromosome 22. In addition to the complete bcr gene, the human genome contains 3 bcr-related genes containing the last seven exons of the intact bcr gene. One of these genes was analysed in detail and showed high sequence homology with the latter. The three bcr-related genes were probably derived from the intact gene by subsequent steps of duplication.