The quantification of NADH:ubiquinone oxidoreductase activity relative to citrate synthase activity showed significantly reduced complex I function (Mann–Whitney test: p = 0.003) in the LRRK2+/PD+ group [median: 0.26, interquartile range (IQR): 0.16–0.37] compared to the LRRK2+/PD− group (median: 0.49, IQR: 0.26–0.70) (Figure 1A). Here, CS is linked to Parkinson disease.