We investigated 1406 ER+ early-stage breast cancers with 20 years' long-term clinical follow-up data for pol β, FEN1, APE1, XRCC1, single-strand monofunctional uracil glycosylase-1 (SMUG1), PARP1, ATR, ATM, DNA-PKcs, Chk1, Chk2, p53, breast cancer susceptibility gene 1 (BRCA1), and topoisomerase 2 (TOPO2) expression. The gene discussed is SMUG1; the disease is breast cancer.