This study will consist of a Screening Period, a Baseline period, and Ongoing Assessment Periods. During the Screening Period, subjects will be assessed for study eligibility including verification of existing laboratory evidence of a pathologic mutation at SLC6A8 gene. A comprehensive history and physical and neurological examination, including evaluation of growth and dysmorphic features, will be completed for all subjects. During the Baseline Period, the caregiver will be interviewed and study staff will administer scales/questionnaires at the study site. For purpose of this protocol, a duly authorized patient representative (e.g. parent, legal guardian) will be referred to as a caregiver. Biological, physiological and radiographic assessments, including ERP recordings, MRS, skin biopsy, cerebrospinal fluid, urine and blood will be obtained. During the Ongoing Assessment Period, growth assessments and limited physical and neurological examinations will be performed. All scales/questionnaires will be repeated every 6 months (± 2 weeks) for 24 months either at the study site or with instructions for completion at home.