Tautomycin is a potent and specific protein phosphatase inhibitor that functions as an antifungal antibiotic. Protein phosphatase-1 (Kiapp = 0.16 nM) (PP1) is more potently inhibited by tautomycin than protein phosphatase 2A (Kiapp = 0.4 nM) (PP2). Studies show that in the presence of tautomycin the expression levels of transferring receptor (TFR) increase and it also blocks PDBu-induced down regulation of CD4. Medullary thyroid cancer (MTC) cells treated with tautomycin and tautomycetin, also an antifungal antibiotic, showed suppressed levels of hormonal secretion, decreased neuroendocrine markers, and inhibition of growth through apoptosis. Both compounds, tautomycin and tautomycetin, can inhibit the growth of MTC cells by blocking glycogen synthase kinase-3β. Tautomycin can also activate the Raf-1 pathway, prevent neuroendocrine (NE) production, and decrease carcinoid cell development.