MYH6encodes the alpha heavy chain subunit of cardiac myosin. The gene is located 4kb downstream of the gene encoding the beta heavy chain subunit of cardiac myosin. Mutations in this gene cause familial hypertrophic cardiomyopathy and atrial septal defect 3. Unlike the hemoglobin and immunoglobulin examples in which switches are unidirectional--a gene switched off in a terminally differentiated cell cannot be switched on again--the beta-MYHC gene can be switched on again either spontaneously in older animals or experimentally in response to thyroid hormone depletion/replacement or different mechanical stimuli. The alpha-MYHC gene is expressed also in atrial muscle and the beta-MYHC gene in skeletal slow-twitch muscle.