Some of the early steps in the development of cancer are well known and are consistent among different types of cancers and across various cell types and organs. In vitro tests that examine these known steps in tumor formation have been developed to assess the risk of a substance’s potential carcinogenicity. For example, the cell transformation assay measures both the tumor-initiating activity and the tumor-promoting activity of a substance and serves as a reliable indicator of carcinogenicity without the use of animals.