Normal cells show such a dependence, transformed cells do not. In some cell types, anchorage dependent growth constitutes a further group of regulatory properties. Normal cells exhibit it, transformed cells do not. These features suggest the basis for selecting cell transformants and assaying transformation. Most cell lines are already transformed and by definition not suitable for transformation assays.
Primary cells have a limited life span in culture.
They reach a crisis phase and die after about 50 generations. It is clearly possible to use primary cells in transformation assays but the availability of a characterized cell line behaving in a consistent and identifiable manner is preferable. Todaro and Green developed a mouse fibroblastic cell line called NIH 3T3 which has been extensively used for studying transformation by cellular and viral oncogenes and other agents. They plated mouse fibroblasts at a low density and replated them every 3 days at this low density ensuring that the cells were never in contact. After more than a year they had cells which were capable of indefinite growth in culture but which still exhibited contact inhibition of growth.