MYL7 and hypoplastic left heart syndrome: qPCR analysis confirmed a reduction of ventricular cardiomyocyte transcripts (MYH7, MYL2; Fig. 7d) and revealed increased MYL7/MYL2 and TNNI3/TNNI1 isoform ratio as indicator of cardiac immaturity (Fig. 7d), corroborating our previous findings on maturation defect in HLHS patient-specific CMs40.