In 1996, with support from the Breast Cancer Research Foundation (BCRF) Mary-Claire King and social worker Joan Marks began the New York Breast Cancer Study, which definitively determined that incidence of breast and ovarian cancer was linked to inherited mutations of the genes BRCA1 and BRCA2. The researchers studied women of Ashkenazi Jewish ancestry in New York, a group that was known to have a very high incidence of breast cancer (up to an 80% risk by age 70, compared with 12% in the general population). She has also studied the incidence of breast cancer in Palestinian women.