Histone reader protein required for checkpoint-mediated cell cycle arrest in response to DNA damage within both the S phase and G2/M phases of the cell cycle (PubMed:12475977, PubMed:12499369, PubMed:12551934, PubMed:12607003, PubMed:12607004, PubMed:12607005, PubMed:12611903, PubMed:14695167, PubMed:15201865, PubMed:15377652, PubMed:16049003, PubMed:16377563, PubMed:30898438). Specifically recognizes and binds histone H2AX phosphorylated at 'Ser-139', a marker of DNA damage, serving as a scaffold for the recruitment of DNA repair and signal transduction proteins to discrete foci of DNA damage sites (PubMed:12607005, PubMed:15201865, PubMed:16049003, PubMed:16377563, PubMed:30898438). Also required for downstream events subsequent to the recruitment of these proteins (PubMed:12607005, PubMed:15201865, PubMed:16049003, PubMed:16377563, PubMed:18582474). These include phosphorylation and activation of the ATM, CHEK1 and CHEK2 kinases, and stabilization of TP53/p53 and apoptosis (PubMed:12499369, PubMed:12551934, PubMed:12607004). ATM and CHEK2 may also be activated independently by a parallel pathway mediated by TP53BP1 (PubMed:12499369, PubMed:12551934, PubMed:12607004). Required for chromosomal stability during mitosis by promoting recruitment of TOPBP1 to DNA double strand breaks (DSBs): TOPBP1 forms filamentous assemblies that bridge MDC1 and tether broken chromosomes during mitosis (PubMed:30898438). Required for the repair of DSBs via homologous recombination by promoting recruitment of NBN component of the MRN complex to DSBs (PubMed:18411307, PubMed:18582474, PubMed:18583988, PubMed:18678890). {ECO:0000269|PubMed:12475977, ECO:0000269|PubMed:12499369, ECO:0000269|PubMed:12551934, ECO:0000269|PubMed:12607003, ECO:0000269|PubMed:12607004, ECO:0000269|PubMed:12607005, ECO:0000269|PubMed:12611903, ECO:0000269|PubMed:14695167, ECO:0000269|PubMed:15201865, ECO:0000269|PubMed:15377652, ECO:0000269|PubMed:16049003, ECO:0000269|PubMed:16377563, ECO:0000269|PubMed:18411307, ECO:0000269|PubMed:18582474, ECO:0000269|PubMed:18583988, ECO:0000269|PubMed:18678890, ECO:0000269|PubMed:30898438}. This is the function of MDC1 (mediator of DNA damage checkpoint 1, ENSG00000137337).