Alzheimer's disease: the genetics of risk

Each copy of an epsilon 4 allele of the gene encoding the lipid carrier apoE increases the probability of the disease and shifts its onset to lower ages. Yet the gene does not cause the disease. Thus, although genotyping is highly valuable in diagnosis and drug testing, it cannot offer useful predictions about persons whose cognition is unimpaired. After discovery of a second susceptibility gene, predictability will change profoundly.