Importantly, activation of the KP has been implicated in myriad neurological and neurodegenerative disorders, and elevation in the KYN/TRP and QA/PA ratios have been linked to the development of neurological conditions such as AD30–33, Huntington’s disease34, AIDS/HIV-associated neurocognitive disorder35, Parkinson’s disease36, amyotrophic lateral sclerosis37,38, and multiple sclerosis39. Here, NPPA is linked to AIDS.