The phenomenon of carcinogenesis in cultured cells is discussed more extensively in Chapter 14. The terms transformation and focus formation in cultured cells, the basic end points of determining potential carcinogenicity by this technology, are defined in terms of their use and meaning in Chapter 14. As with other of the short-term tests listed in Table 13.3 and discussed above, determination of the “neoplastic” transformation in cultured cells has also taken the di- rection of the use of a primary (directly from the animal) culture system in which the cells are diploid and normal in all measurable respects. Another direction is the use of a number of cell lines exhibiting aneuploidy but having reasonably defined cultural characteristics. The tech- niques for the latter have been somewhat standardized (Dunkel et al., 1991), and an extensive degree of study has been carried out with primary Syrian hamster embryo (SHE) cells in primary culture for predicting the carcinogenic potential of a variety of chemicals (cf. Isfort et al., 1996; Barrett et al., 1984). While these techniques are relatively straightforward although somewhat more difficult to score in the SHE system, for the most part they suffer from the inability of the cells to metabolize test agents to their ultimate forms. In addition, given the expense required for the establishment and use of tissue culture methodology, this has been a less than popular short- term test for carcinogenic potential. On the other hand, perhaps more than most of the other short-term tests utilized, transformation to neoplasia in vitro in cell culture has found consider- able usefulness in studying the basic cell and molecular aspects of the neoplastic transformation (Chapters 14 and 16).