The c Abl proto oncogene encodes a protein tyrosine kinase that is located in the cytoplasm and nucleus. In chronic myelogenous leukemia and in a subset of acute lymphoblastic leukemias, the c Abl proto oncogene undergoes a (9;22) chromosomal translocation producing a novel rearranged chromosome (the Philadelphia chromosome) As the result of the fusion of c Abl sequences from chromosome 9 to the Bcr gene on chromosome 22. The molecular consequence of this translocation is the generation of a chimeric Bcr/Abl mRNA encoding activated Abl protein tyrosine kinase.Protein tyrosine kinases play important roles in the transduction of extracellular signals. Receptor tyrosine kinases include a myriad of growth factor receptors, which are activated upon ligand binding. Differential binding of adapter proteins may impart some signal specificity, since many of the receptors use the same adapter proteins, and may also link the activation of the receptor to multiple pathways. In contrast, nonreceptor tyrosine kinases are recruited to substrates and or activators by their SH2 and/or SH3 domains. These domains also allow them to interact with activated receptor tyrosine kinases. These kinases are divided into several groups, including the Src, Jak, Abl, Fak, Fps, Csk, Syk and Btk families, and are components of intracellular signaling cascades.