The first identified small molecule inhibitor of BLM helicase (IC50 = 1.8 μM) that is 28-fold selective against the related helicases RECQ1, RECQ5, and E. coli UvrD (IC50s ≥ 50 μM); 25-50 μM dose-dependently inhibits the proliferation of BLM-expressing PSNF5 fibroblast cells.