The kinases called p38 MAP kinases (MAPKs) are intracellular, soluble serine-threonine kinases which belong to a large family of proteins that include the extracellular regulated kinases (ERKs) and c-Jun N-terminal kinases (JNKs). So far four p38 isoforms have been identified, namely p38α, p38β, p38γ and p38δ , a.k.a., p38-α (MAPK14), -β (MAPK11), -γ (MAPK12 / ERK6), and -δ (MAPK13 / SAPK4). The ubiquitously expressed s p38α has been the most extensively studied and is believed to be the most physiologically relevant in the regulation of the inflammatory response. The role of the three other isoforms is not currently well understood, however their primary sites of expression are known. Similar to p38α, p38β is also ubiquitously expressed, while p38γ is expressed predominately in skeletal muscle and p38δ is expressed primarily in the lung, kidney, testis, small intestine and pancreas. The small molecule inhibitor VX-702 is a potent p38 kinase family inhibitor that has been studied for its effects on inflammation and the inflammatory response. VX-702 dose-dependently inhibited the production o IL-6, IL-1β and TNFα (IC(50) = 59, 122 and 99ng/ml, respectively), and in anti-platelet aggregation assays, pre-incubation of platelets with VX-702 (1µM)completely or partially inhibited platelet agonist induced p38 activation (IC(50) = 4 to 20nM). More recently VX-702 has been studied as a potential treatment for rheumatoid arthritis (RA). VX-702 appears to be most effective against MAPK14, followed by MAPK11 and the remaining members of the family, however exact in vitro IC(50) values have not been published. However a large amount of data is available for in vitro growth inhibition assays where VX-702 exhibits poptent activity , having low IC(50) values beginning at ~16nM and ending at < 400µm="" for="" the="" numerous="" cell="" lines="">