Among heavy coffee drinkers, the impact of GRIN2A on PD risk (measured as OR) was 50% greater, and 2 to 5 orders of magnitude more significant (measured as P value) than the strongest associations reported for SNCA, MAPT or HLA. This study is proof of concept that inclusion of environmental factors can help identify disease-associated genes that are missed in SNP-only GWAS. This evidence concerns the gene GRIN2A and Parkinson disease.