Key regulator of TP53BP1 that plays a key role in the repair of double-strand DNA breaks (DSBs) in response to DNA damage: acts by promoting non-homologous end joining (NHEJ)-mediated repair of DSBs (PubMed:23333306). In response to DNA damage, interacts with ATM-phosphorylated TP53BP1, allowing recruitment to DNA DSBs (By similarity). Once recruited to DSBs, RIF1 and TP53BP1 act by promoting NHEJ-mediated repair of DSBs (PubMed:23333306). In the same time, RIF1 and TP53BP1 specifically counteract DSBs resection via homologous recombination (HR) during G1 phase (By similarity). Interacts with TP53BP1 (when phosphorylated by ATM). Following interaction with TP53BP1, recruited to sites of DNA damage, such as double-strand DNA breaks (DSBs). Belongs to the RIF1 family.