BCR (Breakpoint cluster region protein) is a novel 155-165 kDa Ser/Thr protein kinase. It is found in fibroblasts and hematopoietic cells, interacts with 14-3-3, and serves as a natural inhibitor of the growth factor-associated c-Abl nonreceptor protein tyrosine kinase. Human BCR is 1271 amino acids (aa) in length. It contains a coiled-coil dimerization motif (aa 28-68), a Ser/Thr kinase region (aa 176-426), one DH domain (aa 498-691) and a Rho-Gap region (aa 1054-1248). The N-terminal aa 1-902 and 1-927 are known to contribute to the BCR/ABL oncoprotein. BCR likely forms homodimers and 650 kDa homotetramers. An alternate start site exists at Met498, while other isoforms show a deletion of aa 961-1004, plus a 61 and 37 aa substitution for aa 1-488. Over aa 1-426, human BCR is 89% aa identical to mouse BCR.