SLX1 and SLX4 encode a heteromeric structure-specific endonuclease. The Slx1–Slx4 nuclease is active on branched DNA substrates, particularly simple-Y, 5′-flap, or replication forkstructures. It cleaves the strand bearing the 5′ nonhomologous arm at the branch junction and generates ligatable nicked products from 5′-flap or replication fork substrates. Slx1 is the founding member of a family of proteins with a predicted URI nuclease domain and PHD-type zinc finger. This subunit displays weakstructure-specific endonuclease activity on its own, is stimulated 500-fold by Slx4, and requires the PHD finger for activity in vitro and in vivo. Both subunits are required in vivo for resistance to DNA damage by methylmethane sulfonate (MMS).