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 (By similarity). Required for proper crossover distribution through inhibition of crossover formation at the central region of chromosomes (By similarity). Forms a heterodimer with him-18/slx-4. Both the N- and C-terminal regions are required for interaction with him-18. Belongs to the SLX1 family.