The QQ plots for stratified GWAS also demonstrate clearly that GRIN2A is the single primary PD associated locus in heavy coffee drinkers (Figure 2): exclusion of SNCA, HLA and MAPT did not have an impact in heavy drinkers, whereas exclusion of GRIN2A nearly abolished the extreme P values of 10−5–10−6. Here, MAPT is linked to Parkinson disease.