Type I (OMIM 278300) is caused by xanthine dehydrogenase/oxidase deficiency (XDH/XO, EC 1.17.1.4/1.17.3.2) [1]; type II (OMIM 603592) results from a combined deficiency of XDH/XO and aldehyde oxidase (AOX, EC:1.2.3.1) caused by dysfunctional variants in the molybdenum cofactor sulfurase (MOCOS, EC 2.8), which is required for XDH/XO and AOX activity [2]. This evidence concerns the gene MOCOS and hyperinsulinemic hypoglycemia, familial, 4.