NPPA and Huntington disease: Although the activity of the KP is necessary for tryptophan homeostasis, immune system regulation, and NAD synthesis, overactivity of the KP is associated with neurodegenerative disorders, including Parkinson’s disease (Havelund et al., 2017), Huntington’s disease (Mazarei and Leavitt, 2015), multiple sclerosis (Rejdak et al., 2002), amyotrophic lateral sclerosis (Guillemin et al., 2005b), and AD (Widner et al., 2000; Bonda et al., 2010; Gulaj et al., 2010; Campbell et al., 2014; Maddison and Giorgini, 2015).