She was a 32-year-old aerobics instructor from a Dallas suburb — healthy, college-educated, with two young children. Nothing out of the ordinary, except one thing. Her cholesterol was astoundingly low. Her low-density lipoprotein, or LDL, the form that promotes heart disease, was 14, a level unheard-of in healthy adults, whose normal level is over 100. The reason: a rare gene mutation she had inherited from both parents. Only one other person, a young, healthy Zimbabwean woman whose LDL cholesterol was 15, has ever been found with the same mutation. The discovery of the mutation and of the two women with...