1. The first stage of tumor induction by a carcinogen; subtle alteration of cells by exposure to a carcinogenic agent so that they are likely to form a tumor upon subsequent exposure to a promoting agent (promotion). 2. Starting point of replication or translation in macromolecule biosynthesis. 3. Start of chemical or enzymatic reaction. 4. The first step in a chain reaction.
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n.
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ini·ti·a·tion (ĭ-nĭ″she-aґshən) in toxicology, the creation of a small alteration in the genetic makeup of a cell by a low level of exposure to a carcinogen; the cell may later become neoplastic upon repeated exposure to the same carcinogen or exposure to a promoter.Medical dictionary. 2011.