MTHFR and X-linked creatine transporter deficiency: Twenty-one out of 318 patients had urine creatine metabolite panel measurement for monitoring of creatine metabolism for their known inherited metabolic disorders including six patients with creatine transporter deficiency [6] and 15 patients with seven different inherited metabolic disorders (ornithine-δ-aminotransferase deficiency, ornithine transcarbamylase deficiency, citrullinemia type I, arginase deficiency, MELAS, MTHFR deficiency, MAT I/III deficiency) (Fig. 1 and 2, Additional file 1).