Epidemiologic evidence supports the idea of chemically induced carcinogenesis. Chemical carcinogens can react with the cell's DNA to induce certain genetic mutations by altering the function of regulatory genes. Some chemical agents stimulate cell proliferation through increased cell division. Many chemical carcinogens can induce genetic mutations and increase cell proliferation by acting either directly or indirectly. Direct-acting chemicals do not require a chemical transformation to induce carcinogenesis. Indirect-acting chemicals, or procarcinogens, require metabolic conversion in the host to be capable of malignant transformation. Both direct-acting and metabolically converted chemicals have electron-deficient atoms that react with electron-rich sites within the cell, including DNA and RNA. Tobacco and ethanol are examples of chemicals capable of inducing carcinogenesis.