MKI67 and breast cancer: The “Thrombin Inhibition Preoperatively (TIP) in early breast cancer” is a clinical trial aiming to determine whether 14 days of a preoperative oral factor Xa inhibitor (rivaroxaban) in estrogen-receptor-negative early breast cancer patients results in inhibition of tumor proliferation determined by a reduction in tumor Ki67 from baseline (pre-treatment) to 14 days post treatment start (at the time of surgical excision).