Estrogen promotes PAD4 expression in MCF-7 breast cancer cells through both ER-mediated classical and nonclassical pathways [266], and PAD4 can be recruited to the promoter of estrogen-activated genes to inhibit transcriptional activation of ER target genes by antagonizing arginine methyltransferase [14,101]; in addition, it can synergistically activate the oncogene c-Fos [121] and Elk-1, thereby promoting cancer progression. This evidence concerns the gene ESR1 and breast cancer.