The MYHCs have been divided into 9 to 11 classes. Class II, or conventional, MYHCs include the extensively studied group of sarcomeric MYHCs that self associate to form filaments and function enzymatically to promote contraction in striated (cardiac and skeletal) muscles. Muscle myosin is a heterohexamer consisting of 2 myosin heavy chains and 2 associated nonidentical pairs of myosin light chains. The 7 MYHC isoforms that predominate in mammalian skeletal muscles include 2 developmental isoforms, MYHC-embryonic (MYH3) and MYHC-perinatal (MYH8); 3 adult skeletal muscle isoforms, MYHC IIa (MYH2), MYHC IIb (MYH4), and MYHC IIx/d (MYH1); and MYHC-beta/slow (MYH7), which is also expressed in cardiac muscle. MYHC-extraocular (MYH13) is expressed primarily in extrinsic eye muscles.