The XPC-HR23B complex recognizes various helix-distorting lesions in DNA and initiates global genome nucleotide excision repair. Here we describe a novel functional interaction between XPC-HR23B and thymine DNA glycosylase (TDG), which initiates base excision repair (BER) of G/T mismatches generated by spontaneous deamination of 5-methylcytosine. XPC-HR23B stimulated TDG activity by promoting the release of TDG from abasic sites that result from the excision of mismatched T bases. In the presence of AP endonuclease (APE), XPC-HR23B had an additive effect on the enzymatic turnover of TDG without significantly inhibiting the subsequent action of APE. Our observations suggest that XPC-HR23B may participate in BER of G/T mismatches, thereby contributing to the suppression of spontaneous mutations that may be one of the contributory factors for the promotion of carcinogenesis in xeroderma pigmentosum genetic complementation group C patients.