A 17-year-old boy collapses during the fourth quarter of a high school basketball game and is successfully defibrillated from ventricular fibrillation. He had noticed chest tightness and lightheadedness during sprints for several months. He takes no medications and does not use stimulants. His father died suddenly while jogging at age 41. His pulse is 64/min and blood pressure is 118/72 mm Hg. A harsh crescendo-decrescendo systolic murmur at the left sternal border gets louder when he stands from a squat and softer with handgrip. ECG shows deep, narrow Q waves in the lateral leads and voltage criteria for left ventricular hypertrophy. Echocardiography shows a septum 24 mm thick with systolic anterior motion of the mitral valve. The underlying disorder in this patient is most commonly caused by a mutation in a gene encoding which of the following?
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