Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abnormal cell division. The process of carcinogenesis may be divided into at least three stages: initiation, promotion, and progression. The first stage of carcinogenesis, initiation, results from an irreversible genetic alteration, most likely one or more simple mutations, transversions, transitions, and/or small deletions in DNA.