The solute carrier family 16 member 2 gene (SLC16A2, previously known as MCT8) encodes a transmembrane protein that has been implicated in the transport of triiodothyronine into neurons. The SLC16A2 gene is located at Xq13.2 and contains six exons. Various types of mutations have been associated with Allan-Herndon-Dudley syndrome, which is generally characterized by severe mental retardation, childhood hypotonia, and generalized muscle weakness (Schwartz CE et al. Am J Hum Genet. 2005;77:41-53).