A Stat3 phosphorylation inhibitor. LLL12 inhibits STAT3 phosphorylation (Tyr705) and STAT3 activities and down-regulates STAT3 downstream targets; inhibits proliferation, colony formation, and cell migration and induces apoptosis in various human breast and pancreatic cancer cells as well as in glioblastoma cells. LLL12 has minimal apoptotic effects on normal human cells.