Myosin regulatory light polypeptide 9 is a protein that in humans is encoded by the MYL9 gene. Myosin regulatory subunit that plays an important role in regulation of both smooth muscle and nonmuscle cell contractile activity via its phosphorylation. Myosin, a structural component of muscle, consists of two heavy chains and four light chains. MYL9 is a myosin light chain that may regulate muscle contraction by modulating the ATPase activity of myosin heads. MYL9 binds calcium and is activated by myosin light chain kinase.