is a gene encoding a myosin heavy chain beta (MHC-β) isoform (slow twitch) expressed primarily in the heart.In early fetal development, MHC-β is predominately expressed in the ventricles, while MHC-α is predominantly expressed in the atria. In healthy adult hearts, MHC-α is predominantly expressed in the atria (>90%), while MHC-β (~50%) is expressed in the atria of failing adult hearts. In vitro studies have demonstrated that newly formed cardiac myocytes express MHC-β, and after prolonged (1-5 weeks) contractile activity MHC-α becomes detectable. An allelic variant of this gene is associated with approximately 40% of the cases of Hypertrophic cardiomyopathy (HCM). This condition is an autosomal-dominant disease, in which a single copy of the variant gene causes enlargement of the left ventricle of the heart.