Hahn et al. (2002) report that the mutation of the creatine-transporter (SLC6A8) gene was found in patients with mild retardation and other behavioral problems, and Rosenberg et al. (2004) described one patient with X-linked mental retardation that coincided with a large deletion of the SLC6A8 and other patients with missense mutations of the gene [28,29]. This evidence concerns the gene SLC6A8 and Intellectual disability.