After disease onset, in the case that GBA mutations are benign in LRRK2 Parkinson disease (PD), according to an inert hypothesis for LRRK2/GBA carriers, treatment responses for effective LRRK2-targeting therapies would be identical for carriers of LRRK2 and LRRK2/GBA carriers. Alternatively, in the masked hypothesis for carriers of LRRK2/GBA, LRRK2 variations are biologically masking the effects of GBA variations in disease progression, and LRRK2-targeting therapy would unmask the effects of GBA variations to match disease progression and cognitive decline associated with GBA variations.