Nonmuscle myosin is an actin-based motor protein essential to cell motility, cell division, migration, adhesion, and polarity. The holoenzyme consists of two identical heavy chains and two sets of light chains. The light chains (MLCs) regulate myosin II activity and stability. The heavy chains (NMHCs) are encoded by three genes, MYH9, MYH10, and MYH14, which generate three different nonmuscle myosin II isoforms, IIa, IIb, and IIc, respectively. While all three isoforms perform the same enzymatic tasks, binding to and contracting actin filaments coupled to ATP hydrolysis, their cellular functions do not appear to be redundant and they have different subcellular distributions. The carboxy-terminal tail domain of myosin II is important in isoform-specific subcellular localization. Research studies have shown that phosphorylation of myosin IIa at Ser1943 contributes to the regulation of breast cancer cell migration.