Several methylation markers for breast cancers have been investigated, such as GSTP1, RASSF1A and RARβ2, and hypermethylation of these genes has been reported as, respectively, 17–48 %, 43–90 % and 26–78 % in breast cancer tissues and as 2–3 %, 3–8 % and 0 % in normal breast tissues (Yamamoto et al. 2012; Jung et al. 2013; Hagrass et al. 2014; Pirouzpanah et al. 2015), indicating the equally high sensitivity and specificity of TRIM9 as a methylation marker for breast cancer. Here, GSTP1 is linked to breast cancer.