APEX1 is a multifunctional 36.5 kDa protein encoded on chromosome 14 that serves as a central mediator of cellular responses to oxidative stress 1. Its primary function involves DNA repair through the base excision repair (BER) pathway, where it acts as an apurinic/apyrimidinic endonuclease that catalyzes hydrolytic incision of the phosphodiester backbone adjacent to AP sites, generating single-strand breaks with appropriate 3' and 5' ends for downstream repair 2. APEX1 possesses 3'-5' exonuclease and 3' phosphodiesterase activities essential for short-patch BER completion. Beyond DNA repair, APEX1 exerts reversible nuclear redox activity to regulate transcriptional factor DNA-binding affinity through controlled reduction of their DNA-binding domains 1. Clinically, APEX1 expression correlates with cancer progression and treatment resistance. APEX1 gene amplification and overexpression in osteosarcoma independently predicted disease-free survival and metastatic risk 3. In ulcerative colitis, APEX1 expression in intestinal epithelium drives neutrophil infiltration and barrier dysfunction through NF-κB-dependent CXCL1 production; APEX1 inhibition reduced inflammatory severity 4. APEX1 polymorphisms show population-specific associations with cancer susceptibility, with certain variants offering protective effects in breast cancer 5. Targeting APEX1's redox activity represents a promising therapeutic strategy for both cancer and inflammatory disorders.