Endonuclease that cooperates with the MRE11-RAD50-NBN (MRN) complex in DNA-end resection, the first step of double-strand break (DSB) repair through the homologous recombination (HR) pathway. Functions downstream of the MRN complex and ATM, promotes ATR activation and its recruitment to DSBs in the S/G2 phase facilitating the generation of ssDNA. Homodimer; dimerizes via the coiled coil domain. Interacts with the MRN complex; the interaction links DNA sensing to resection (By similarity). Interacts with samhd1 (PubMed:29670289). Associates with sites of DNA damage in S/G2 phase. Binds chromatin. The damage-recruitment motif is required for DNA binding and translocation to sites of DNA damage. The PXDLS motif binds to a cleft in CtBP proteins. Belongs to the COM1/SAE2/CtIP family.