Two systems are essential in humans for genome integrity, DNA repair and apoptosis. Cells that are defective in DNA repair tend to accumulate excess DNA damage. Cells defective in apoptosis tend to survive with excess DNA damage and thus allow DNA replication past DNA damages, causing mutations leading to carcinogenesis. It has recently become apparent that key proteins which contribute to cellular survival by acting in DNA repair become executioners in the face of excess DNA damage. Five major DNA repair pathways are homologous recombinational repair (HRR), non-homologous end joining (NHEJ), nucleotide excision repair (NER), base excision repair (BER) and mismatch repair (MMR). In each of these DNA repair pathways, key proteins occur with dual functions in DNA damage sensing/repair and apoptosis. Proteins with these dual roles occur in: (1) HRR (BRCA1, ATM, ATR, WRN, BLM, Tip60 and p53); (2) NHEJ (the catalytic subunit of DNA-PK); (3) NER (XPB, XPD, p53 and p33(ING1b)); (4) BER (Ref-1/Ape, poly(ADP-ribose) polymerase-1 (PARP-1) and p53); (5) MMR (MSH2, MSH6, MLH1 and PMS2). For a number of these dual-role proteins, germ line mutations causing them to be defective also predispose individuals to cancer. Such proteins include BRCA1, ATM, WRN, BLM, p53, XPB, XPD, MSH2, MSH6, MLH1 and PMS2.