SIBA (5′-Isobutylthio-5′-deoxyadenosine, 5′-Deoxy-5′-isobutylthioadenosine, 5′-Isobutylthioadenosine) is a synthetic analogue of S-adenosylhomocysteine and is a powerful antipro-liferative drug. SIBA is not only an inhibitor of transmethylation but also interferes with polyamine biosynthesis, probably by blocking aminopropyltransferase.