Women live longer than men with Alzheimer’s because their sex chromosomes give them genetic protection from the ravages of the disease. Women get two ‘doses’ of a gene that only exists on the X chromosome. And some people, both male and female, have an especially potent variant of this gene. Long-term studies of older people, many of whom already had mild cognitive impairment, showed women with one or two copies of the variant progressed more slowly toward Alzheimer’s.