With colon cancer, for example, it is known that at least six or seven types of gene are involved, of which only one type is hereditary. The other genes are mutations associated with the external environment.
以大腸癌為例，目前已經找出至少有六、七種基因發生變化，但其中可能只有一個基因來自遺傳，其餘的都是後天環境因素造成基因突變。