SLC6A8, an X-linked gene, encodes the creatine transporter (CRTR) and its mutations lead to cerebral creatine (Cr) deficiency which results in mental retardation, speech and language delay, autistic-like behaviour and epilepsy (CRTR-D, OMIM 300352).
Our findings of a minimum of four pathogenic mutations and two potentially pathogenic mutations indicate that about 1% of males with MR of unknown etiology might have a SLC6A8 mutation.