The lack of a complex V defect in proteomics RCA analysis of both MT-ATP6 cases (VC20 and UDP1) prompted us to analyse additional patients with known pathogenic variants affecting complex V. Supporting cohort probands SC1 and SC2 were two unrelated infants presenting with lactic acidosis, cardiac abnormalities (persistent patent ductus arteriosus, ventriculo-septal defect and cardiomyopathy), mild dysmorphic facial features, hypospadias, hyperammonaemia and 3-methyl-glutaconic aciduria. Here, MT-ATP6 is linked to cardiomyopathy.