Carcinogenesis, also known as oncogenesis or tumorigenesis, is the process by which normal, healthy cells transform into cancer cells. It is a multistep process that involves modification and mutation to genes that regulate normal cellular function, including cell growth and division. Carcinogens are substances, organisms, or agents capable of causing cancer by interacting with a cells DNA to produce mutations. The target of the carcinogenic factor is chromosomal DNA, where a lesion that can be repaired or that will be reproduced occurs. Most human cancers result from exposure to environmental (or exogenous) carcinogens, such as tobacco smoke, ultraviolet rays, and certain viruses, but other carcinogens that cause malignant transformation include a broad group of factors from within the body, termed endogenous factors. The process of carcinogenesis is formally a multistage process, since between the initial carcinogenic stimulus and the final manifestation of cancer, there are several stages, including initiation, promotion, and progression. The transformation of a normal cell to a malignant cell is thought to be due to successive and cumulative exposures to carcinogens and other factors over the course of decades.