Many tumor suppressors play an important role in the DNA damage pathway. Zinc 
finger protein 668 (ZNF668) has recently been identified as one of the potential 
tumor suppressors in breast cancer, but its function in DNA damage response is 
unknown. Herein, we report that ZNF668 is a regulator of DNA repair. ZNF668 
knockdown impairs cell survival after DNA damage without affecting the ATM/ATR 
DNA-damage signaling cascade. However, recruitment of repair proteins to DNA 
lesions is decreased. In response to IR, ZNF668 knockdown reduces Tip60-H2AX 
interaction and impairs IR-induced histone H2AX hyperacetylation, thus impairing 
chromatin relaxation. Impaired chromatin relaxation causes decreased recruitment 
of repair proteins to DNA lesions, defective homologous recombination (HR) 
repair and impaired cell survival after IR. In addition, ZNF668 knockdown 
decreased RPA phosphorylation and its recruitment to DNA damage foci in response 
to UV. In both IR and UV damage responses, chromatin relaxation counteracted the 
impaired loading of repair proteins and DNA repair defects in ZNF668-deficient 
U2OS cells, indicating that impeded chromatin accessibility at sites of DNA 
breaks caused the DNA repair defects observed in the absence of ZNF668. Our 
findings suggest that ZNF668 is a key molecule that links chromatin relaxation 
with DNA damage response in DNA repair control.