Siomycin is a macrocyclic antibiotic with potent and selective antibacterial activity discovered in 1969. Siomycin is a potent inhibitor of the oncogenic transcription factor, FoxM1. The reduced transcription activity is reflected in the down regulation of protein and mRNA levels of FoxM1 and effects on down stream genes, Cdc25B, survivin and CENPB. In vitro, siomycin inhibits FoxM1-induced cell growth on soft agar and selectively kills transformed but not normal cells.