MYL9, also known as MLC2, is one of the numerous regulatory myosin light chains. Regulatory myosin light chains regulate contraction in smooth muscle and non-muscle cells via phosphorylation by myosin light chain kinase(MLCK). Phosphorylation of regulatory myosin light chains is catalyzed by MLCK in the presence of calcium and calmodulin and it increases the actin-activated myosin ATPase activity, thereby regulates the contractile activity.