In particular, comparing with the individuals of OPRM1 hypomethylation in PBL DNA and regular sports, we found that less frequent sports (<1 time/week) combined with OPRM1 hypermethylation in PBL DNA increased breast cancer risk with a strong association (OR = 4.114, 95%CI: 2.357–7.181). Here, OPRM1 is linked to breast carcinoma.