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. Forms a heterodimer with SLX4. Belongs to the SLX1 family. Found in a segmental duplication on p arm of chromosome 16 giving rise to two identical copies of this gene sharing exons with SULT1A3 and SULT1A4.