LRRK2 and Parkinson disease: <h4>Background</h4>Gain-of-kinase-function variants in LRRK2 are a leading cause of monogenic Parkinson's disease (PD).<h4>Objectives</h4>We tested the functional impact of a novel LRRK2 variant p.V1447L identified in a young-onset PD patient in vivo in peripheral blood, as well as in a robust cellular assay, alongside other variants in close proximity to V1447.<h4>Methods</h4>We measured LRRK2-dependent Rab10 phosphorylation in neutrophils and monocytes of a LRRK2 p.V1447L carrier with PD.