A third yet indirect observation is that countries with higher prevalence of DRB1*13:02 display lower rates of dementia.23 Given that DRB1*13:02 differs from DRB1*13:01 by a single amino acid that is known to influence the peptide binding groove and in turn the repertoire of presented peptides66,67 these observations imply that antigen presentation may be the underlying link between certain HLA alleles and AD. This evidence concerns the gene HLA-DRB1 and Alzheimer disease.