Scientists have identified a genetic form of Alzheimer’s disease in older people, pinpointing a gene of which some people carry two copies. The gene, called APOE4, is not only a risk factor but an underlying cause for the disease when found twice in a person’s DNA, researchers say. Finding a way to target APOE4 in treatment is crucial because Leqembi, the only drug found to slow the disease, causes dangerous side effects in people with the gene. The Associated Press has more.