In humans, LRRK2 is principally expressed in blood (gtexportal.org), and LRRK2 protein levels are elevated in classical (e.g., pro-inflammatory) monocyte populations as measured in idiopathic PD patients (lacking pathogenic LRRK2 mutations) in two different study populations in Europe and the United States [7, 8]. Here, LRRK2 is linked to Parkinson disease.