Other genetic studies of Parkinson's patient cohorts have found common variants within the LRRK2 and Rab29 genes that function coordinately to increase Parkinson's risk, as human genetic variants at these loci impact Parkinson's risk non‐additively (MacLeod et al, 2013; Pihlstrom et al, 2015). This evidence concerns the gene RAB29 and Parkinsonism.