Essential for cell cycle arrest at the G1 and G2 stages following DNA damage by X-, and UV-irradiation, or inactivation of DNA ligase. Plays a role in the response to DNA damage (PubMed:9153313, PubMed:9407031). Interaction with rad4 via its phosphorylation sites in the N-terminus couples the DNA checkpoint apparatus to chromatin via interaction of its C-terminal BRCT domains with epigenetic modifications on histones H4 and H2A, respectively, in the G1/S phase of the cell cycle, and facilitates recruitment of the checkpoint kinase chk1 (PubMed:15550243, PubMed:16778077, PubMed:18826944, PubMed:20679485, PubMed:22792081). Homodimer. Dimerization is mediated via the BRCT domain (PubMed:16778077, PubMed:18676809). Interacts (via BRCT domain) with rad3 (PubMed:14739927). Interacts with rad4 (via BRCT1,2 domains) (PubMed:9407031, PubMed:14739927); a single rad4 molecule interacts simultaneously with both Thr-187 phosphorylation sites in a crb2 dimer (PubMed:24074952). Interacts (via Tudor domain) with histone H4K20me2 (PubMed:18826944, PubMed:17190600). Interacts (via BRCT dmain) with histone H2AS128ph (gamma-H2A) (PubMed:20679485, PubMed:18676809). Interacts with chk1 (PubMed:9407031, PubMed:14739927, PubMed:22792081). Interacts with sad1 (PubMed:14655046). Recruited to sites of DNA damage, such as double stand breaks. The Tudor-like region mediates binding to H4K20me2. Phosphorylation of Thr-73 and Ser-80 by rad3/ATM promotes interaction with chk1 (PubMed:22792081). Phosphorylation at Thr-187 is dependent on phosphorylation at Thr-215 and Thr-235. Phosphorylation at Thr-215 and Thr-235 may prime the non-canonical Thr-187 site for cdc2/CDK phosphorylation (PubMed:24074952). Not resistant to the microtubule depolymerizing drug thiabendazole (TBZ). DNA damage sensitive in response to ionizing radiation (IR), ultraviolet (UV), hydroxyurea (HU) and camptothecin (CPT).