The APOE gene encodes Apolipoprotein E (ApoE). ApoE is essential for packaging and carrying cholesterol and fats through the bloodstream. There are three common alleles of the APOE gene: APOE e2, APOE e3 and APOE e4. APOE e4 is the strongest known genetic risk factor for Alzheimer’s, while APOE e3 is the most common allele, and APOE e2 appears to have a neuroprotective effect against Alzheimer’s.