Serum tumor biomarkers for breast cancer, such as carcinoembryonic antigen (CEA), cancer antigen 125 (CA125), and cancer antigen 15-3 (CA15-3) have low sensitivity[2] and so are typically used for follow-up monitoring rather than for early diagnosis.[3,4] There is therefore a need for new serum tumor biomarkers or for the development of new diagnostic methods for breast tumor screening. Here, MUC16 is linked to breast cancer.