SLC6A8, also known as CRTR, CT1, or CRT, belongs to the Na+- and Cl–- dependent neurotransmitter transporter family and transports creatine in the brain and muscles. Since muscle cells do not synthesize creatine, they must take it up via SLC6A8. Missense mutations in the SLC6A8 gene result in the X-linked creatine deficiency syndrome, which is characterized by mental retardation and physical developmental disorders due to a defect in creatine metabolism.