A chemical-genetic method for the generation of target-specific protein kinase inhibitors has been developed recently. This strategy utilizes a functionally silent active-site mutation to sensitize a target kinase to inhibition by a small molecule that does not inhibit wild-type kinases. Tyrosine and serine/threonine kinases are equally amenable to the drug-sensitization approach, which has been used to generate selective inhibitors of mutant Src-family kinases, Abl-family kinases, cyclin-dependent kinases, mitogen-activated kinases, p21-activated kinases and Ca(2+)/calmodulin-dependent kinases. The designed inhibitors are specific for the sensitized kinase in a cellular background where the wild-type kinase has been inactivated. By these means, kinase-sensitization has been used systematically to generate and analyze conditional alleles of several yeast protein kinases in vivo.