APOE and early-onset autosomal dominant Alzheimer disease: In humans, the Apolipoprotein E (APOE) gene has 3 variants (e2, e3, e4); the e4 allele has been subject to considerable research interest due to it being a well-established risk factor for late-onset Alzheimer’s disease (AD)1 with e4 carriers being approximately 3 times more likely to develop AD relative to e3 carriers2.