FEN1-IN-4 is a potent and selective inhibitor of flap endonuclease 1 (FEN1), an enzyme involved in DNA replication and repair processes. Its primary application lies in cancer research, where it is used to target cancer cells with defective DNA repair pathways, particularly those exhibiting synthetic lethality with homologous recombination deficiencies such as in BRCA-mutated tumors. By inhibiting FEN1, FEN1-IN-4 disrupts base excision repair and Okazaki fragment maturation, leading to accumulation of DNA damage and cell death in rapidly dividing tumor cells. It serves as a valuable tool compound in studies exploring DNA repair inhibition as a therapeutic strategy, especially in combination with other DNA-damaging agents like chemotherapy or PARP inhibitors. Its use is currently limited to preclinical research, aiding in the validation of FEN1 as an oncology drug target.