ACTN2 and Arrhythmia: Some patients present with dilated cardiomyopathy with ventricular and cardiomyocyte hypertrophy and interstitial fibrosis starting at 7 years of age,4 and others present with biventricular hypertrophy and arrhythmias between 10 and 40 years.5–8 Additional phenotypes include progressive skeletal myopathy with onset ranging from infancy to adulthood.9,10 In this case, the proband’s clinical presentation combined with the identical ACTN2 mutation detected in his affected brother provides compelling evidence for the variant’s pathogenicity.