Thus far, we have only been considering measurement of LRRK2 in blood for the purposes of target engagement, but there has been considerable effort put into measurement of LRRK2 levels and LRRK2 function in blood for the purpose of patient stratification or testing the hypothesis that PD patients without LRRK2 mutations have elevated LRRK2 function that contributes to PD pathogenesis. Here, LRRK2 is linked to Parkinson disease.