Soon after the identification of monocarboxylate transporter 8 (MCT8) as a highly-specific transmembrane TH transporter (5), inactivating mutations in the human SLC16A2 gene encoding MCT8 were linked to the altered TH levels detected in AHDS patients (3, 4), now often referred to as the MCT8 deficiency syndrome. The gene discussed is SLC16A2; the disease is Allan-Herndon-Dudley syndrome.