Catalytic subunit of the SLX1-SLX4 structure-specific endonuclease that resolves DNA secondary structures generated during DNA repair and recombination. Has endonuclease activity towards branched DNA substrates, introducing single-strand cuts in duplex DNA close to junctions with ss-DNA. Has a preference for 5'-flap structures, and promotes symmetrical cleavage of static and migrating Holliday junctions (HJs). Resolves HJs by generating two pairs of ligatable, nicked duplex products. {ECO:0000255|HAMAP-Rule:MF_03100, ECO:0000269|PubMed:19595721, ECO:0000269|PubMed:19596235, ECO:0000269|PubMed:19596236}. This is the function of ENSG00000181625 (SLX1B, structure-specific endonuclease subunit SLX1B).