Tunicamycin is a mixture of Tunicamycins A, B, C and D which has been widely used in the study of glycoprotein synthesis in various biological systems. Tunicamycin inhibits GNPTAB (GlcNAc phosphotransferase, GPT) and inhibts the formation of N-glycosidic linkages in glycoprotein synthesis. Tunicamycin has also been reported to have a dose-dependent inhibition of DNA synthesis, to inhibit protein glycosylation, to suppress the S-phase of the cell cycle, and to arrest the cell cycle in late G1. As a member of a family of antibiotics produced by Streptomyces lysosuperficus, this compound is noted to be active in vitro against gram-positive bacteria, fungi, yeasts and viruses. During protein glycosylation, tunicamycin is noted to be an inhibitor of the transfer of saccharide moieties to dolichol during dolichol-linked glycoprotein synthesis. Dose-dependent inhibition of DNA synthesis may be related to the alteration of glycoproteins, which thereby affects the transport of thymidine into cells. Additionally, tunicamycin has been reported to prevent cell cycle progression in primary cultures of rat glial cells, as well as inhibit lipid-mediated protein glycosylation in chick or mouse fibroblasts in a dose-dependent manner.A competitive cell cycle inhibitor