We found weak evidence that increased serum furin reduces breast cancer risk (OR 0.81, 95% CI = 0.67–0.99, P = 0.03 per SD increase in serum furin), and that increased glycoprotein Nmb reduces breast cancer risk (OR 0.88, 95% CI = 0.78–0.99, P = 0.04 per SD increase in serum glycoprotein Nmb). Here, FURIN is linked to breast carcinoma.