HDAC9 and lung cancer: To determine whether promoter hypermethylation caused the inhibition of XRCC3 and XRCC5 expression in the HPV16-positive lung cancer cells, we analyzed the methylation status of XRCC3 and XRCC5 and the expression of the methylation-associated proteins histone deacetylases (HDAC), DNMT1, and DNMT3b in B[a]P-treated HPV-positive and -negative lung cancer cells.