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, D-loops, replication forks, nicked Holliday junctions and also intact Holliday junctions with a reduced efficiency. May be required in mitosis for the processing of stalled or collapsed replication fork intermediates. Plays a role in DNA repair and in genotoxic stress-induced homologous recombination (HR) in somatic cells. Mediates a subset of meiotic recombination events that are insensitive to crossover interference (By similarity). Forms a heterodimer with MUS81. Belongs to the EME1/MMS4 family.