A protein kinase is an enzyme that modifies proteins by transferring a phosphate group from ATP. This phosphorylation creates a phosphoprotein with altered protein activity, often acting as an on/off switch for protein functions. Protein kinases regulate cell functions like signal transduction, cell cycle, metabolism, differentiation, proliferation, and apoptosis. Protein kinases act on the amino acids serine, threonine, and tyrosine. A protein kinase inhibitor is a compound that blocks the action of one or more protein kinases.

Protein kinase dysfunction is connected to a wide variety of human diseases and conditions including cancer, inflammatory conditions, autoimmune disorders, and cardiac diseases. Protein kinase inhibitors are important research tools to study cell functions and human diseases. In some cases protein kinase inhibitors block cell proliferation, as in certain disease conditions. Sigma-Aldrich has a broad range of protein kinase inhibitors for use in the research of protein and cell functions.