SLC16A2 and Cognitive impairment: Its importance to human CNS development has been highlighted by discoveries of different mutations within the MCT8 gene (SLC16A2) in subjects with a variety of X-linked mental retardation syndromes, characterized by severe psychomotor and cognitive impairment and accompanied by elevated serum free T3 concentrations but normal or low free T4 concentrations (Dumitrescu et al. 2004, Friesema et al. 2004, Schwartz et al. 2005).