Inhibits Stat3.Stat3 complex formation and Stat3-DNA binding activity in vitro (IC₅₀ = 86 ± 33 µM) and Stat3-dependent transcriptional activities. 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.