The regulation of cell growth, differentiation and programmed death is coordinated by several sets of proteins that comprise essential signal transduction pathways. Many of these key regulatory proteins are encoded by proto-oncogenes, which can be activated (altered) to change the typical cell program to one of abnormal cell growth and unregulated development. Proteins encoded by proto-oncogenes include growth factors and other ligands, receptor proteins, tyrosine kinases, various regulatory proteins (i.e. GTPases) and transcription factors. Together these proteins comprise the basic elements of cell signaling pathways; altered expression or mutation of one or more of these components can lead to oncogenic growth. Non-receptor (i.e. cytoplasmic, nuclear) tyrosine kinases such as c-Abl and Src play key roles in the regulation of cell proliferation, differentiation, apoptosis, cell adhesion and stress responses. Alteration of the corresponding c-Abl and Src proto-oncogenes is associated