Gliotoxin is a potent crystalline antibiotic substance produced during the growth of the imperfect fungus Gliocladium jimbriatum. It is a natural mycotoxin with immunosuppressive and antimicrobial activity. Gliotoxin acts as NOTCH2 transactivation inhibitor in human neoplasias. It induced growth inhibition and apoptosis in multiple colorectal cancer cell lines with mutations of the Wnt signaling pathway. Gliotoxin stimulates the apoptosis of human and rat hepatic stellate cells and enhances the resolution of liver fibrosis in rats. Gliotoxin is a dual inhibitor of farnesyltransferase and geranylgeranyltransferase I with pronounced antitumor activity and favorable toxicity profile against breast cancer in vitro and in vivo.