SLC22A5 and systemic primary carnitine deficiency disease: Primary carnitine deficiency (PCD), also known as primary carnitine uptake defect (CUD) or carnitine transporter deficiency (CTD), is caused by defective organic cation transporter 2 (OCTN2), which is an organic cation/carnitine transporter encoded by SLC22A5; mutations in SLC22A5 result in increased urinary excretion of carnitine; carnitine deficiency in blood, tissues, and cells; and defective fatty acid oxidation.