OPRM1 and breast cancer: DNA methylation was quantified from d = 6, 4, or 4 CpG sites located in OPRM1, TLR4, and LINE1, respectively, in n = 70 patients with persistent pain and in n = 70 patients assigned to the “non-persistent pain” phenotype group based on the 3-year follow-up data after breast cancer surgery (Fig. 1a).