Catalytic component of a structure-specific DNA repair endonuclease responsible for the 5-prime incision during DNA repair, and which is essential for nucleotide excision repair (NER) and interstrand cross-link (ICL) repair. {ECO:0000269|PubMed:10413517, ECO:0000269|PubMed:11790111, ECO:0000269|PubMed:19596235, ECO:0000269|PubMed:24027083, ECO:0000269|PubMed:32034146, ECO:0000269|PubMed:8797827}. This is the function of ERCC4 (ERCC excision repair 4, endonuclease catalytic subunit, Ensembl gene identifier ENSG00000175595).