APOE genotype is represented by combination of e2 (episilon 2),e3 and e4. Each individual will have one of the following combinations: e2/e2, e2/e3, e2/e4, e3/e3, e3/e4, e4/e4. Although the order between two genotypes for each person doesn't matter, they are represented in the order of e2>e3>e4. Therefore, APOE A1 is the first APOE genotype of a given individual and the number 3 means that the first genotype is e3 for that person.