MYH6 and hypoplastic left heart syndrome: Resulting cardiomyocytes showed dysmorphic sarcomeres (Box 1) that caused decreased atrial contractility and compensatory increased expression of MYH7. When the ‘phenotypically normal’ parental iPSCs were edited by using CRISPR/Cas9 to contain the MYH6-R443P variant, the resulting cardiomyocytes reproduced the HLHS phenotype of the proband.