Except for the few diseases for which a change in a single SNP is critical (such as APOE4 variant in Alzheimer’s disease), for the vast majority of diseases, the odds ratio (OR; the ratio of the chance of a given event occurring in a given group as compared to the same event occurring in another group) for a single SNP is below 1.5, showing lack of association. Here, APOE is linked to early-onset autosomal dominant Alzheimer disease.