APOE and early-onset autosomal dominant Alzheimer disease: ApoE also bears significance as the most important genetic risk factor for the sporadic form of Alzheimer's disease (AD) as carriers of the APOEε4 allele (encoding apoE4) are at a significantly higher risk of developing AD than individuals having the common APOEε3 gene variant (encoding apoE3) (Corder et al., 1993).