MDC1 (mediator of DNA damage checkpoint 1) is a scaffold protein essential for the DNA damage response (DDR) and genome integrity maintenance 1. As a histone reader protein, MDC1 recognizes phosphorylated histone H2AX at sites of DNA double-strand breaks (DSBs), serving as a critical platform for recruiting DNA repair and signaling proteins 1. This recruitment enables phosphorylation and activation of ATM, CHEK1, and CHEK2 kinases, leading to checkpoint-mediated cell cycle arrest in S and G2/M phases and stabilization of TP53 for apoptosis if damage is irreparable 1. MDC1 promotes homologous recombination repair by facilitating MRN complex recruitment to DSBs 1. During mitosis, MDC1 maintains chr6 stability by recruiting TOPBP1 to form filamentous assemblies that tether broken chr6 until subsequent repair 2. Beyond its canonical DDR role, MDC1 regulates RNA polymerase II transcription elongation and serves as a co-regulator of nuclear receptor activity 3. Clinically, MDC1 functions as a tumor suppressor; its loss contributes to cancer development and progression while predicting sensitivity to chemotherapy and RNAPII inhibitors 13. Deleterious MDC1 mutations are associated with genomic instability and malignancies including melanoma 4. MDC1 emerges as a promising therapeutic target for radiosensitization and improving cancer treatment outcomes 5.