Regulatory 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. Forms a heterodimer with slx1. Phosphorylated in response to DNA damage. Belongs to the SLX4 family.