This functional connection between KMO and KYNA affects brain physiology [46,47,48,49] and may also have significant pathophysiological relevance since both impaired KMO activity and abnormal KYNA levels are seen in several neurological and psychiatric disorders including HD [50], epilepsy [51], schizophrenia [52,53,54], and bipolar disorder [35,55]. Here, KMO is linked to Huntington disease.