SPR and Platelet storage pool disease: Furthermore, the discovery of patients with sepiapterin reductase deficiency (SPD) who show normal urinary excretion of pterins supports the proposal that BH4 biosynthesis from 6‐pyruvoyltetrahydropterin could be compensated by carbonyl and/or aldose reductases in the case of complete hSPR defect and suggests the possible role of the non‐enzymatic activity of SPR in the disease.