AbstractPhosphorylation is the most common and important mechanism of acute and reversible regulation of protein function. Studies of mammalian cells metabolically labeled with [32P]orthophosphate suggest that as many as one‐third of all cellular proteins are covalently modified by protein phosphorylation. Protein phosphorylation has an important role in essentially all aspects of cell biology. Most polypeptide growth factors (platelet‐derived growth factor and epidermal growth factor are among the best studied) and cytokines (e.g., interleukin 2, colony stimulating factor 1, and γ‐interferon) stimulate phosphorylation upon binding to their receptors. Induced phosphorylation in turn activates cytoplasmic protein kinases, such as Raf, the activators of the mitogen‐activated protein (MAP) kinases SEK and MEK, the MAP kinases ERK, JNK, and p38, the Janus/JAK kinases, the p21 activated kinases (PAKs), and the phosphatidylinsoitil 3'‐kinase‐activated kinase, protein kinase B/Akt. Additionally, in all nucleated organisms, cell cycle progression is regulated at both the G1/S and the G2/M transitions by cyclin‐dependent protein kinases. These kinases regulate the G1/S transition by the phosphorylation of cell cycle regulators such as Rb protein and the G2/M transition through the phosphorylation of nuclear lamins and histones.