ERCC2 and xeroderma pigmentosum group D: Excision repair cross-complementing 1 (ERCC1) and excision repair cross-complementing 2/xeroderma pigmentosum group D (ERCC2/XPD) are considered two key rate-limiting enzymes in the multistep NER process, because of their pivotal role in the recognition and removal of damaged nucleotides.