PTPS deficiency (PTPSD) caused by mutations in the PTS gene, is the most common disorder in BH4D, followed by DHPR deficiency (DHPRD) caused by defects in the quinoid dihydropteridine reductase (QDPR) gene (Opladen et al., 2012). Here, QDPR is linked to dihydropteridine reductase deficiency.