FEN1-IN-1 (Compound 1) is an inhibitor of Flap endonuclease 1 (FEN1), binding to its active site and inhibiting activity partly by coordinating with Mg²⁺ ions. By blocking FEN1, it initiates a DNA damage response, activating the ATM checkpoint pathway, histone H2AX phosphorylation, and FANCD2 ubiquitination in mammalian cells.