See related article, pages 1549–1552 Hypertrophic cardiomyopathy (HCM) is a common inherited disorder with an estimated prevalence of ≥1 in 500 worldwide. The disease is inherited in families in an autosomal dominant fashion and is usually caused by mutations in genes encoding contractile proteins such as cardiac β-myosin heavy chain ( MYH7 ), cardiac troponin T ( TNNT2 ), cardiac myosin binding protein-C ( MYBPC3 ), cardiac troponin I, α-myosin heavy chain, cardiac α-actin, α-tropomyosin, titin, myosin regulatory light chain, and myosin essential light chain. Several HCM disease genes remain to be identified. The most obvious clinical manifestation of HCM is left ventricular hypertrophy. Although such hypertrophy is classically asymmetrical with prominent involvement of the interventricular septum, both concentric and apical hypertrophy can also occur. In fact, affected individuals may not exhibit any hypertrophy. Other clinical features are similarly variable in their expression and include sudden cardiac death, heart failure, arrhythmias, stroke, heart block, and infective endocarditis. Some patients remain asymptomatic throughout their lifetime. Many HCM patients have no or only minor symptoms, and asymptomatic affected children and adolescents are often diagnosed during family screening after another family member comes to medical attention.1 The average annual risk of sudden cardiac death in a HCM patient is 1%,2 and in high risk patients, prophylactic defibrillator implantation can be lifesaving. Given marked variation in penetrance and expressivity, early diagnosis and reliable prognostic tools are crucial for primary prevention and proper followup of affected individuals and their family members. Current guidelines recommend that all first-degree relatives of an individual affected by HCM should be clinically evaluated by history, physical examination, electrocardiography, and echocardiography. With increased understanding of the molecular genetic causes of HCM and advances in modern laboratory technology, clinical genetic testing for HCM has become increasingly feasible.3 The GeneTests database (http://www.ncbi.nlm.nih.gov/sites/GeneTests) currently lists 5 …