Key implications are: (1) circulating sitosteryl hexoside associations with motor severity may be protective in some contexts (PD, PD-LRRK2) yet pathological in others (GBA1) depending on lysosomal competence and sterol species; (2) functional studies in genotype-stratified models (GBA1 and LRRK2 cellular and animal systems) are essential to determine whether manipulation of sitosteryl hexoside (or modulation of sterol metabolism) will be beneficial or harmful. This evidence concerns the gene GBA1 and Parkinson disease.