IMPORTANCE The genetic variant MYBPC3 Δ25bp occurs in 4%of South Asian descendants, with an estimated 100 million carriers worldwide. MYBPC3 Δ25bp has been linked to cardiomyopathy and heart failure. However, the high prevalence of MYBPC3 Δ25bp suggests that other stressors act in concert with MYBPC3 Δ25bp . OBJECTIVE To determine whether there are additional genetic factors that contribute to the cardiomyopathic expression of MYBPC3 Δ25bp . DESIGN, SETTING, ANDPARTICIPANTS South Asian individuals living in the United States were screened for MYBPC3 Δ25bp , and a subgroup was clinically evaluated using electrocardiograms and echocardiograms at Loyola University, Chicago, Illinois, between January 2015 and July 2016. MAIN OUTCOMES AND MEASURES Next-generation sequencing of 174 cardiovascular disease genes was applied to identify additional modifying gene mutations and correlate genotype-phenotype parameters. Cardiomyocytes derived from human-induced pluripotent stem cells were established and examined to assess the role of MYBPC3 Δ25bp . RESULTS In this genotype-phenotype study, individuals of South Asian descent living in the United States from both sexes (36.23%female) with a mean population age of 48.92 years (range, 18-84 years) were recruited. Genetic screening of 2401 US South Asian individuals found an MYBPC3 Δ25bp carrier frequency of 6%. A higher frequency of missense TTN variation was found in MYBPC3 Δ25bp carriers compared with noncarriers, identifying distinct genetic backgrounds within the MYBPC3 Δ25bp carrier group. Strikingly, 9.6%of MYBPC3 Δ25bp carriers also had a novel MYBPC3 variant, D389V. Family studies documented D389V was in tandem on the same allele as MYBPC3 Δ25bp , and D389V was only seen in the presence of MYBPC3 Δ25bp . In contrast to MYBPC3 Δ25bp , MYBPC3 Δ25bp/D389V was associated with hyperdynamic left ventricular performance (mean [SEM] left ventricular ejection fraction, 66.7 [0.7%]; left ventricular fractional shortening, 36.6 [0.6%]; P < .03) and stem cell-derived cardiomyocytes exhibited cellular hypertrophy with abnormal Ca2+ transients. CONCLUSIONS AND RELEVANCE MYBPC3 Δ25bp/D389V is associated with hyperdynamic features, which are an early finding in hypertrophic cardiomyopathy and thought to reflect an unfavorable energetic state. These findings support that a subset of MYBPC3 Δ25bp carriers, those with D389V, account for the increased risk attributed to MYBPC3 Δ25bp . © 2018 American Medical Association. All rights reserved.