Most Alzheimer’s disease cases are sporadic and late-onset (typically found in those 65 and older), where heritability is estimated to be between 60 and 80%.17 There are, a number of identified common variants, most notably the e4 allele of the APOE (apolipoprotein) gene, which accounts for ∼5% of Alzheimer’s disease heritability, plus about 20 additional loci that account for up to 30% Alzheimer’s disease heritability.18 It is likely that the remaining heritability is the result of the combination of a great many (1000s to 100 000s) common variants, each contributing a very small effect. This evidence concerns the gene APOE and early-onset autosomal dominant Alzheimer disease.