Catalytic subunit of a heterodimeric 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 (Potential). Has a preference for replication forks over 5' flap structures or Holliday junctions and shows much lower activity toward 3' flap structures (PubMed:22927825). Required for proper crossover distribution through inhibition of crossover formation at the central region of chromosomes (PubMed:22927825, PubMed:23874210). Forms a heterodimer with him-18/slx-4. Both the N- and C-terminal regions are required for interaction with him-18. Reduced brood size, increased embryonic lethality and increased larval arrest (PubMed:22927825, PubMed:23874210). Increased number of rad51 foci in meitotic and mitotic nuclei and increased germ cell apoptosis (PubMed:22927825). Hypersensitivity to ultraviolet C, nitrogen mustard and camptothecin but not to gamma irradiation (PubMed:22927825). Increased chromosomal abnormalities including increased intra- and inter-bivalent chromatin bridges (PubMed:23874210). Increased meiotic crossover frequency in the center of chromosomes III, IV, V and X with decreased frequency in the arm regions of chromosomes III, IV, V and X (PubMed:22927825). Belongs to the SLX1 family.