Interacts with MUS81 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 MUS81. Recruited to regions of DNA damage in S-phase cells. Belongs to the EME1/MMS4 family.