LRRK2 and Parkinson disease: Finally, as it should be clear from the literature reviewed here, while the field has made great advances in the use of LRRK2-targeted biomarkers as measures of target engagement (i.e., for small molecule inhibitors of LRRK2 kinase), much work remains in optimizing the interpretation of these outcome measures for use in staging the disease, tracking progression, predicting pheno-conversion (in carriers of specific mutations), or as a tool to confirm the diagnosis of PD.