A reversible, cell-permeable inhibitor of Src-family tyrosine kinases that have been mutated, by a single base substitution, to become 'analog sensitive' (as), as compared to the wild-type kinase; inhibits v-Src-as1, with an I338G substitution, preferentially over v-Src (IC50 = 1.5 nM versus 1.0 μM, respectively); used to elucidate functions of several kinases in both mammalian and yeast systems