The BCR/ABL oncogene product is one of the genes associated with and possibly responsible for human leukemias. Chronic myelogenous leukemia and Philadelphia chromosome-positive acute lymphoblastic leukemia are associated with p210BCR/ABL and p185BCR/ABL, respectively. Abelson murine leukemia virus encodes the related oncogene product, v-Abl, and also causes pre-B-cell leukemia. In this article, recent advances in understanding the function of these oncogenes as well as the function of normal counterparts, c-Abl and Bcr, are discussed. Intracellular signaling events initiated from these oncogene products are emphasized. The possibilities are also discussed that inhibition of apoptosis and altered adhesive properties to the bone marrow microenvironment by BCR/ABL might contribute to disease initiation.