LRRK2 and Parkinson disease: In this review, we describe (1) the clinical and neuropathological similarities and differences of LRRK2 parkinsonism with sporadic or idiopathic PD (iPD); (2) what genetic modifiers of LRRK2, including Mendelian PD genes, GWAS risk factors, and polygenic risk scores, modify disease risk; (3) what LRRK2 mutations are associated with pleomorphic LRRK2 neuropathology; (4) what environmental modifiers affect LRRK2 parkinsonism.