Chronic myelogenous leukemia (CML) is a disease characterized by the presence of p210 bcr-abl , a chimeric protein with tyrosine kinase activity. Substrates for p210 bcr-abl are likely to be involved in the pathogenesis of CML. Here we describe the purification, cDNA cloning, and characterization of a 56-kDa tyrosine phosphorylated protein, p56 dok-2 (Dok-2), from p210 bcr-abl expressing cells. The human dok-2 cDNA encodes a 412amino acid protein with a predicted N-terminal pleckstrin homology

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... omain as well as several other features of a signaling molecule, including 13 potential tyrosine phosphorylation sites, six PXXP motifs, and the ability to bind to p120 RasGAP . Dok-2 was shown to be 35% identical to p62 dok-1 , a recently identified RasGAP binding protein from CML cells, and analysis of the expressed sequence tag data base revealed the presence of at least four additional proteins containing a Dok homology sequence motif. Dok mRNAs were primarily expressed in tissues of hematopoietic origin. These findings strongly suggest that a family of Dok-related proteins exists that bind to RasGAP and may mediate the effects of p210 bcr-abl in CML. Human chronic myelogenous leukemia (CML) 1 is a myelo-proliferative disease (reviewed in Ref. 1) characterized by the presence of a chromosomal translocation known as the Philadelphia chromosome. Cells from CML patients contain a t(9;22) translocation in which the 5Ј exons of the bcr (breakpoint cluster) gene on chromosome 22 are fused to the c-abl protooncogene on chromosome 9 (2). The most common fusion generates the chimeric protein p210 bcr-abl responsible for CML.