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. May serve as a scaffold for the recruitment of DNA repair and signal transduction proteins to discrete foci of DNA damage marked by 'Ser-139' phosphorylation of histone H2AX. Also required for downstream events subsequent to the recruitment of these proteins. These include phosphorylation and activation of the ATM, CHEK1 and CHEK2 kinases, and stabilization of TP53 and apoptosis. ATM and CHEK2 may also be activated independently by a parallel pathway mediated by TP53BP1. Homodimer. Interacts with several proteins involved in the DNA damage response, although not all these interactions may be direct. Interacts with H2AX, which requires phosphorylation of H2AX on 'Ser-139'. Interacts with the MRN complex, composed of MRE11, RAD50, and NBN. Interacts with CHEK2, which requires ATM-mediated phosphorylation of 'Thr-68' within the FHA domain of CHEK2. Interacts constitutively with the BRCA1-BARD1 complex, SMC1A and TP53BP1. Interacts with ATM and FANCD2, and these interactions are reduced upon DNA damage. Also interacts with the PRKDC complex, composed of XRCC6/KU70, XRCC5/KU80 and PRKDC/XRCC7. This interaction may be required for PRKDC autophosphorylation, which is essential for DNA double strand break (DSB) repair. When phosphorylated by ATM, interacts with RNF8 (via FHA domain). Interacts with CEP164. When phosphorylated, interacts with APTX (via FHA-like domain). Associated with chromatin. Relocalizes to discrete nuclear foci following DNA damage, this requires 'Ser-139' phosphorylation of H2AX. Colocalizes with APTX at sites of DNA double-strand breaks. Highly expressed in testis. Tandemly repeated BRCT domains are characteristic of proteins involved in DNA damage signaling. In MDC1, these repeats are required for localization to chromatin which flanks sites of DNA damage marked by 'Ser-139' phosphorylation of H2AX. Phosphorylated upon exposure to ionizing radiation (IR), ultraviolet radiation (UV), and hydroxyurea (HU). Phosphorylation in response to IR requires ATM, NBN, and possibly CHEK2. Also phosphorylated during the G2/M phase of the cell cycle and during activation of the mitotic spindle checkpoint. Phosphorylation at Thr-4 by ATM stabilizes and enhances homodimerization via the FHA domain. Sumoylation at Lys-1840 by PIAS4 following DNA damage promotes ubiquitin-mediated degradation. Ubiquitinated by RNF4, leading to proteasomal degradation; undergoes 'Lys-48'-linked polyubiquitination. Truncated C-terminus.