Our mix-and-match cultures of healthy iPSC-MSNs and -CNs with GBA-N370S iPSC-DaNs allow us to understand the effect of mutant DaNs on MSNs and the associated role in the progression of PD among GBA1 mutation carriers, a component that would have been masked if investigating only all-GBA cortico-striato-nigral cultures11. Here, GBA1 is linked to Parkinson disease.