Characteristic of chronic myelogenous leukemia (CML) is the presence of the chimeric p210^bcr-ablprotein possessing elevated protein tyrosine kinase activity relative to normal c-abl tyrosine kinase. Hematopoietic progenitors isolated from CML patients in the chronic phase contain a constitutively tyrosine-phosphorylated protein that migrates at 62 kDa by SDS–PAGE and associates with the p120 ras GTPase-activating protein (GAP). We have purified p62^dokfrom a hematopoietic cell line expressing p210^bcr-abl. p62^dokis a novel protein with features of a signaling molecule. Association of p62^dokwith GAP correlates with its tyrosine phosphorylation. p62^dokis rapidly tyrosine-phosphorylated upon activation of the c-Kit receptor, implicating it as a component of a signal transduction pathway downstream of receptor tyrosine kinases.