In this context, Braak’s hypothesis (12) connects the onset of PD to the alleles HLA-DRB1*15:01, -DRB1*04:02:01, -DQA1*03, and -DQB1*03:02:01 that are associated with Crohn’s disease, colitis or celiac disease (63–65), and that we found were significant (p<0.05) in the PPMI cohort (Tables 3–5; Supplementary Table 4). Here, HLA-DQA1 is linked to Crohn disease.