Background：

A reciprocal translocation between chromosomes 22 and 9 produces the Philadelphia chromosome， which is often found in patients with chronic myelogenous leukemia. The chromosome 22 breakpoint for this translocation is located within the BCR gene. The translocation produces a fusion protein which is encoded by sequence from both BCR and ABL， the gene at the chromosome 9 breakpoint. Although the BCR-ABL fusion protein has been extensively studied， the function of the normal BCR gene product is not clear. The protein has serine/threonine kinase activity and is a GTPase-activating protein for p21rac. Two transcript variants encoding different isoforms have been found for this gene.