APEX1 and head and neck squamous cell carcinoma: NER) (ERCC1, ERCC2/XPD, XPA, and XPC) and BER (APEX1, XRCC1) genes were downregulated in patients with HNSCC compared with the HC, whereas double-strand break repair (MRE11A, RAD50, RAD51, XRCC2) and mismatch repair (MLH1, MSH2, MSH3) genes were up-regulated (15).