VE-821 is a potent and selective inhibitor of the ataxia telangiectasia and Rad3-related (ATR) kinase, a key protein involved in the DNA damage response pathway. It is primarily used in cancer research to sensitize tumor cells to DNA-damaging agents such as chemotherapy and radiation. By inhibiting ATR, VE-821 prevents cancer cells from repairing DNA damage, leading to increased cell death, especially in cells with defective DNA repair mechanisms. This compound shows promise in enhancing the efficacy of existing anticancer treatments and is widely used in preclinical studies to explore synthetic lethality strategies, particularly in tumors with mutations in related pathways such as ATM deficiency. Its application supports the development of targeted therapies that exploit genomic instability in cancer.