Interacts with EME1 and EME2 to form a DNA structure-specific endonuclease with substrate preference for branched DNA structures with a 5'-end at the branch nick. Typical substrates include 3'-flap structures, replication forks and nicked Holliday junctions. May be required in mitosis for the processing of stalled or collapsed replication forks. May self-associate. Interacts with EME1, EME2 and CHEK2. Interacts with BLM, and this interaction may stimulate the endonuclease activity of MUS81. Interacts with SLX4/BTBD12; this interaction is direct and links the MUS81-EME1 complex to SLX4, which may coordinate the action of the structure-specific endonuclease during DNA repair. Interacts with DCLRE1B/Apollo. Interacts with RECQL5; this interaction stimulates mitotic DNA synthesis (By similarity). Recruited to foci of DNA damage in S-phase cells. Belongs to the XPF family.