The dramatic success of the first approved protein kinase inhibitor imatinib (Gleevec) for the treatment of chronic myelogenous leukemia has fueled an explosion in kinase inhibitor research in both the pharmaceutical industry and academia.However, due to the large size of the protein kinase superfamily and the fact that most kinase inhibitors bind in the highly conserved ATP-binding pocket, the selectivity of kinase inhibitors is extremely difficult to achieve.