Ab-2 labels myosin fast-twitch (Type II) heavy-chain myosin. Myosin is a 480kDa protein known to interact with actin in muscle and in non-muscle cells. It contains two identical heavy chains (200kDa each) and four light chains (15-26kDa). Myosin molecules consist of two major regions: tails (rod) and heads; they aggregate into filaments through the tail region and interact with actin and with ATP through the head region. Multiple forms of myosin heavy chains exist for each muscle type-skeletal, cardiac, smooth and non-muscle isomyosin forms exist in different types of skeletal muscle, depending on the physiological function of the muscle. These are designated at type I (slow-twitch) and type II (fast-twitch).