The most active compound, Stattic, inhibited Stat3 binding to a cognate pY-peptide ligand, inhibited ligand-mediated Stat3 phosphorylation and nuclear translocation, reduced Stat3 binding to DNA and induced apoptosis of breast cancer cells with constitutively activated Stat3 in the 5–20 μM range. The gene discussed is STAT3; the disease is breast cancer.