NSC 74859 (S3I-201) inhibits Stat3-Stat3 complex formation and Stat3 DNA-binding (IC50 =86 ±33μM) and transcriptional activities. Furthermore, S31-201 inhibits growth and induces apoptosis preferentially in tumor cells that contain persistently activated Stat3. NSC 74859 (S3I-201) inhibits the expression of the Stat3-regulated genes encoding cyclin D1, BcI-xL, and survivin and inhibits the growth of human breast tumors in vivo. Therefore the antitumor activity of S31-201 is mediated in part through inhibition of aberrant Stat3 activation and provide the proof-of-concept for the potential clinical use of Stat3 inhibitors such as NSC 74859 (S31-201) in tumors harboring aberrant Stat3.