SLC6A8 and metabolic disease: Cerebral Cr deficiency syndromes (CCDS), due to AGAT, GAMT or CrT deficiencies, are metabolic diseases characterized by brain Cr deficiency, causing a range of clinical features such as severe neurodevelopmental delays, intellectual disability (ID) with speech delay, behavioral disturbances, motor dysfunction, and seizures (Item et al., 2001; van de Kamp et al., 2013; Salomons et al., 2001; Schulze and Braissant, 2017; Stöckler et al., 1994; Stöckler-Ipsiroglu and van Karnebeek, 2014).