SB 203580 is a specific inhibitor of p38α and p38β which suppresses downstream activation of MAPKAP kinase-2 and heat shock protein 27. At low concentrations, it does not inhibit JNK activity. SB 203580 is a specific inhibitor of p38α and p38β which suppresses downstream activation of MAPKAP kinase-2 and heat shock protein 27. At low concentrations, it does not inhibit JNK activity.