In the current study, we have used immunohistochemistry (IHC) to examine the expression of MRN and other factors involved in DSB repair (BRCA1, BRCA2, ATM, CHK2, ATR, CHK1, pCHK1, RAD51, γH2AX, RPA1, RPA2, DNA-PKcs), RECQ helicases (BLM, WRN, RECQ1, RECQL4, RECQ5), nucleotide excision repair (ERCC1) and base excision repair (SMUG1, APE1, FEN1, PARP1, XRCC1, Pol β) in a large clinical cohort of 1650 breast cancers. Here, FEN1 is linked to breast cancer.