Regulatory subunit that interacts with and increases the activity of different structure-specific endonucleases. Has several distinct roles in protecting genome stability by resolving diverse forms of deleterious DNA structures. Component 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. Interacts with the structure-specific RAD1-RAD10 endonuclease and promotes RAD1-RAD10-dependent 3'-non-homologous tail removal (NHTR) during repair of double-strand breaks by single-strand annealing. SLX4 also promotes recovery from DNA-alkylation-induced replisome stalling during DNA replication by facilitating the error-free mode of lesion bypass. This does not require SLX1 or RAD1-RAD10, but probably RTT107. Forms a heterodimer with SLX1. Interacts with RAD1; catalytic subunit of the RAD1-RAD10 endonuclease. Interacts with RTT107. Phosphorylated by ATR (MEC1) and ATM (TEL1) upon DNA damage. This appears to be required for the function with the RAD1-RAD10 endonuclease. Present with 274 molecules/cell in log phase SD medium. Belongs to the SLX4 family.