(A) The chemical structure of Gleevec. The drug can be given by mouth; it has side effects but they are usually quite tolerable. (B) The structure of the complex of Gleevec (solid green object) with the tyrosine kinase domain of the Abl protein (ribbon diagram), as determined by X-ray crystallography. (C) Gleevec sits in the ATP-binding pocket of the tyrosine kinase domain of Bcr-Abl and thereby prevents Bcr-Abl from transferring a phosphate group from ATP onto a tyrosine residue in a substrate protein. This blocks onward transmission of a signal for cell proliferation and survival. (B, from T. Schindler et al., Science 289:1938–1942, 2000. © AAAS.)