MEK5 is a dual specificity protein kinase belonging to thr Ser/Thr protein kinase family, (MAP kinase kinase family). It is activated by phosphorylation on Ser/Thr by MAP kinase kinases and interacts specifically with ERK5, and not with another MAP kinase like P38. This kinase specifically interacts with and activates MAPK7/ERK5. This kinase itself can be phosphorylated and activated by MAP3K3/MEKK3, as well as by atypical protein kinase C isoforms (aPKCs). It is not phosphorylated by RAFA, RAFB or RAFC and it may interact with GTPases such as CDC42. The signal cascade mediated by this kinase is involved in growth factor stimulated cell proliferation and muscle cell differentiation. MEK5 is expressed in many adult tissues and is most abundant in heart and skeletal muscle.