XRCC1 and breast cancer: In breast cancer patients, somatic changes may be present in other genes involved in DNA repair: (1) homologous recombination (ATM, ATR, CHEK1, CHEK2, BARD1, RAD51, NBS1, PALB2, FANCD2), (2) non-homologous end joining (DNA-PK, KU70/80), (3) mismatch repair (MLH1, MSH1, MSH6, PMS2), (4) base excision repair (APE1, XRCC1, ERCC2) [40].