VPS35 and Parkinson disease: To test if our assay can determine LRRK2 activity in PD patients with the VPS35 D620N mutation, we measured Rab10-Thr73 phosphorylation occupancies in neutrophils isolated from three heterozygous patients, as well as three age-matched idiopathic PD patients and three nonPD controls (see Experimental Procedure, supplemental Table S4).