Apolipoprotein E is a blood protein that has to do with cholesterol transport in the blood. The gene for APOE is located in chromosome no. 19. There are three different gene variants (alleles) for ApoE that encode the corresponding three protein variants of ApoE called ApoE2, ApoE3 and ApoE4. There is a clear link between the gene variant for APOE-4 and the onset of Alzheimer's disease.