The importance of MTH transport by monocarboxylate transporter 8 (MCT8, SLC16A2), in human neurodevelopment is highlighted by the severe global neurological impairment observed in subjects with the rare human disease Allan-Herndon-Dudley-Syndrome (AHDS) (13, 14). The gene discussed is SLC16A2; the disease is Allan-Herndon-Dudley syndrome.