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 simple Y, 5'-flap and replication fork-like structures. It cleaves the strand bearing the 5'-non-homologous arm at the branch site junction and generates ligatable, nicked products from the 5'-flap or replication fork substrates. Plays a critical role in maintaining the integrity of the ribosomal DNA (rDNA) loci, where it has a role in re-starting stalled replication forks. Has Holliday junction resolvase activity in vitro. Forms a heterodimer with SLX4. Belongs to the SLX1 family.