Creatine transporter deficiency is an X-linked disorder caused by to mutations in the creatine transporter gene (SLC6A8; CTR1) localized to Xq28. Males can present with speech and developmental delay, seizures and hypotonia. Carrier females can have a mild intellectual impairment and problems with social skills. Males show a significant elevation in urinary creatine excretion and may have a mild elevation of plasma creatine. Creatine uptake studies or molecular analyses are recommended to confirm positive biochemical findings.