<span class="paragraphSection"><div class="boxTitle">Abstract</div>Nucleotide excision repair (NER) is the key pathway for the removal of DNA damage induced by UV irradiation and chemotherapeutic reagents. Protein–protein interactions are crucial for the dynamic and coordinated assembly of the proteins involved in DNA lesions. Here we focus on the role of interactions between the multi-subunit helicase/translocase complex TFIIH and the 3′ endonuclease XPG. We show that XPG interacts with the p62 and XPD subunits of TFIIH through its long spacer region bridging its split active site. We show that interactions between three acidic regions of XPG and the Pleckstrin homology (PH) domain of p62 are of moderate importance for NER, while defects in the interactions with XPD fail to pull-down TFIIH and strongly reduce NER activity. These p62 and XPD interface mutations additively reduce NER activity. Unexpectedly, we show that these interactions did not impair the recruitment of XPG but instead were defective in the formation of a catalytically competent NER complex and in triggering the incision 5′ to the lesion by ERCC1–XPF. Our studies provide fundamental insights into how interactions between TFIIH and XPG contribute to the NER pathway and, more generally, how modular protein–protein interactions control each step along the NER reaction coordinate.</span>