(A) Reversible phosphorylation and (B) kinase drug discovery. For explanation, see text. DualPase, dual-specificity phosphatases; DualPK, dual-specificity kinases; PI, phosphatidylinositol; PIP, phosphatidylinositolphosphate; PI3K, phosphatidylinositol kinases; STPase, Ser/Thr-specific phosphatases; STPK, Ser/Thr-specific kinases; TPK, Tyr-specific kinases.