Using high‐performance liquid‐chromatography (HPLC) with tandem mass spectroscopy (MS/MS) quantification methods, we measured levels of six KP metabolites (Figure 1)—TRP, L‐kynurenine (L‐KYN), KYNA, 3‐HK, anthranilic acid (AA) and QUIN—in CSF and blood plasma, and applied a predefined statistical analysis plan to investigate the hypothesis that KMO activity is important to HD pathogenesis. The gene discussed is KMO; the disease is Huntington disease.