For example, GBA, SCARB2, and SMPD1, which are collectively implicated in PD risk, are also jointly involved in sphingolipid metabolism; fly homologs of all 3 genes (Gba1b, Dsb, and CG15533, respectively) were identified in our screen as synergistic, loss-of-function enhancers of αSyn. The gene discussed is SMPD1; the disease is Parkinson disease.