A structure-specific endonuclease, cleaves 5' of ds/ssDNA interfaces in 3' flap structures, although it also cuts bubble, Y-DNA structures and mobile and immobile Holliday junctions. Cuts preferentially after pyrimidines, may continue to progressively cleave substrate upstream of the initial cleavage, at least in vitro. May be involved in nucleotide excision repair. Nuclease function requires interaction with the PCNA heterotrimer. Inhibited by SSB, even in the presence of PCNA. Interacts with PCNA heterotrimer via PCNA1 and PCNA3.