Finally, our results indicate that GSLs, including glucosylceramide (GlcCer) and galactosylceramide (GalCer), may serve as potential treatment-responsive biomarkers of LRRK2 inhibition in the CNS based on our analysis of CSF from LRRK2-PD patients dosed with a LRRK2 inhibitor in a phase 1b study. This evidence concerns the gene LRRK2 and Parkinson disease.