To investigate whether MSCs are able to transfer mitochondria in two model systems of mitochondrial NDUFS4 deficiency (e.g. patient-derived fibroblasts with pathogenic NDUFS4 mutations and mouse fibroblast with NDUFS4 knockout), we performed co-culture studies of MSCs with fibroblast cell lines. The gene discussed is NDUFS4; the disease is hyperinsulinemic hypoglycemia, familial, 4.