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Xanthatin, a xanthanolide sesquiterpene lactone isolated from Xanthium strumarium L. (Asteraceae), has prominent anti-tumor activity. Initial mechanism of action studies suggested xanthatin triggered activation of Wnt/β-catenin. We examined the effects of xanthatin on signaling pathways in A459 lung cancer cells and mouse embryonic fibroblasts to ascertain requirements for xanthatin-induced cell death and tumor growth in xenografts. Genetic inactivation of GSK-3β, but not the related isoform GSK-3α, compromised xanthatin cytotoxicity while inactivation of β-catenin enhanced xanthatin-mediated cell death. These data provide insight into how xanthatin and related molecules could be effectively targeted toward certain tumors.