The members of the solute carrier family 22 (SLC22) belong to the organic cation/anion/zwitterion transporter family. SLC22A5, which is also known as OCTN2, is expressed in the muscle, heart, and kidney. It is mainly involved in the transportation of carnitine in a sodium-dependent manner. Primary systemic carnitine deficiency is caused by mutations in the SLC22A5 gene. A mouse with a spontaneous mutation in the slc22a5 gene, which is designated as juvenile visceral steatosis (JVS), develops an enlarged fatty liver, steatosis of other organs, and hypertrophic cardiomyopathy, and eventually dies at 4 to 5 weeks unless carnitine is administered. Defects in SLC22A5/OCTN2 may also lead to increased susceptibility to Crohn’s disease.