However, this raises the bigger unanswered question of where LRRK2 G2019S carriers—who meet clinical diagnostic criteria for PD but lack evidence of α-synuclein seeding—fit in the new biological definitions of PD that are likely to emerge, especially as the causal role of α-synuclein aggregates in PD is still being debated.71,72. This evidence concerns the gene LRRK2 and Parkinson disease.