RECQL5-IN-1 (Compound 4a) acts as an orally effective RECQL5 inhibitor (targeting both enzymatic and nonenzymatic domain). RECQL5-IN-1 is a potent inhibitor of RECQL5 helicase activity (IC50=46.3 nM). RECQL5-IN-1 inhibits RECQL5-WT cells and RECQL5-KO2 cells with IC50s of 4.8 μM and 19.6 μM, respectively. RECQL5-IN-1 can be used for the research of breast cancer.