Catalytic domain of the Serine/Threonine Kinase, p38 Mitogen-Activated Protein KinaseSTKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. p38 kinases are mitogen-activated protein kinases (MAPKs), serving as important mediators of cellular responses to extracellular signals. They function in the regulation of the cell cycle, cell development, cell differentiation, senescence, tumorigenesis, apoptosis, pain development and pain progression, and immune responses. p38 kinases are activated by the MAPK kinases MKK3 and MKK6, which in turn are activated by upstream MAPK kinase kinases including TAK1, ASK1, and MLK3, in response to cellular stresses or inflammatory cytokines. p38 substrates include other protein kinases and factors that regulate transcription, nuclear export, mRNA stability and translation. p38 kinases are drug targets for the inflammatory diseases psoriasis, rheumatoid arthritis, and chronic pulmonary disease. Vertebrates contain four isoforms of p38, named alpha, beta, gamma, and delta, which show varying substrate specificity and expression patterns. p38alpha and p38beta are ubiquitously expressed, p38gamma is predominantly found in skeletal muscle, and p38delta is found in the heart, lung, testis, pancreas, and small intestine. The p38 subfamily is part of a larger superfamily that includes the catalytic domains of other STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.