Two salient features have emerged: First, there are no reliable clinical measures or tests to identify a LRRK2 mutation carrier from idiopathic late-onset PD, short of genetic testing.13 In clinical populations, many LRRK2 carriers fail to report a family history of disease and thus are understood as sporadic cases.14 This is owing, in part, to the second feature critical for understanding LRRK2 in PD: Pathogenic mutations are not fully penetrant. Here, LRRK2 is linked to Onset.