Primary deficiency is due to impaired cellular carnitine transport associated with mutations in the genes of organic cation transporters for carnitine encoded by SLC22A4 and SLC22A5, especially given their prevalence, or by mutation of SLC22A5. Primary carnitine deficiency presents symptoms of cardiomyopathy, skeletal myopathy, and hypoglycemia with extremely low carnitine concentrations in blood and tissues. Here, SLC22A5 is linked to skeletal muscle disorder.