Typically, mutations that cause cancer are divided into mutations caused by hereditary factors and those caused by environmental factors. Recently, it has been hypothesized that there is a third source of mutations; these mutations appear as a result of random errors that occur during normal replication of stem cell DNA. Based on this hypothesis, it was concluded that the formation of about 67% of all mutations is not caused by exposure to any mutagens. In this paper, investigation is conducted to determine which part of mutations is due to the action of mutagens. The mechanisms of the formation of targeted base substitution mutations, targeted insertions, targeted deletions and targeted complex mutations are analyzed. The mechanisms of formation of untargeted, targeted delayed and untargeted delayed base substitution mutations are analyzed. The analysis is based on polymerase-tautomeric models for formation of various types of mutations. It was shown that all analyzed types of mutations are caused by mutagens. It is concluded that the prevention and treatment of cancer must necessarily include the removal of these mutagens from organisms. This will be the most effective, safe and cheapest way to prevent and possibly treat cancer.