In terms of explaining why women of the same age differ in risk, the known susceptibility genes, including BRCA1, BRCA2, PALB2, CHEK2, ATM, and TP53 play more important roles at younger ages; the proportion of the total breast cancer familial variance explained by these genes was 46% at age 20–29 years and decreased steadily with age thereafter. The gene discussed is BRCA1; the disease is breast cancer.