In AD, the high odds ratios of risk factors such as the common APOE4-4 genotype or rare variants in TREM2, SORL1 or ABCA7 in EOAD (for review, see [113]) suggest that they contribute significantly to the genetic component of EOAD in a complex determinism, including sporadic and familial presentations. This evidence concerns the gene TREM2 and Alzheimer disease.