Mutation of the leucine-rich repeat kinase 2 gene (LRRK2) causes familial Parkinson disease (PD), but new research indicates that wild-type LRRK2 is also involved in idiopathic PD. In the new study, proximity ligation assays were developed and used to analyse LRRK2 activity in post-mortem brain tissue from patients with idiopathic PD and rat models of the disease. In human and animal tissue, LRRK2 activity in nigrostriatal dopamine neurons was greater in individuals with disease than in controls. Furthermore, in wild-type cultured human cells and in one rat model of PD, activation of LRRK2 was prevented by treatment with LRRK2 inhibitors, suggesting that these drugs could be beneficial not only for patients with LRRK2 mutations, but also for those with idiopathic PD and no mutations in LRRK2.