Potent, cellular STAT3 inhibitor that inhibits Stat3·Stat3 complex formation and Stat3 DNA-binding and transcriptional activities (IC50 value 86 μm for in vitro Stat3–Stat3:DNA disruption). S3I 201 inhibits growth and induces apoptosis preferentially in tumor cells that contain persistently activated Stat3. Additionally, S3I 201 inhibits the expression of the Stat3-regulated genes encoding cyclin D1, Bcl-xL, and survivin and inhibits the growth of human breast tumors in vivo. S3I 201 showed cytotoxic activity against a wide variety of cancer cell lines (IC50 values ranging from 37.9 to 82.6 μm) through inhibition of the reductases P5, protein disulfide isomerase (PDI), thiol-disulfide oxidoreductase ERp57, and/or Trx.