Compared to other BH4 deficiencies, DHPR‐deficient patients show a higher frequency of severe neurological symptoms, including hypotonia, movement disorders (mainly dystonia), microcephaly, epilepsy, the brain atrophy caused by the BH2 accumulation, and the inhibition of nitric oxide synthase and aromatic acid hydroxylases due to DHPR deficiency (Crabtree et al., 2009). The gene discussed is QDPR; the disease is microcephaly.