The rodent fibroblast systems described above have provided sensitive and rapid biological assays to characterize the properties of normal and mutated Ras proteins. Furthermore, these assays have provided in vitro systems to measure the ability of other cellular components to modulate Ras signal transduction and transformation. However, while these assays provide an excellent measure of Ras-transforming activity, the fact that these cells are of fibroblastic origin, and can be transformed by a single hit, indicates that caution should be used in extrapolating observations from NIH 3T3 transformation assays to the situation in human tumors. Therefore, using human epithelial cell-based assays that more closely approximate the cell types where mutated ras alleles are most frequently detected may provide more realistic assays for examining the biochemical and biological consequences of aberrant Ras function in human tumors. Nevertheless, despite these cautions, these rodent transformation assays will continue to be the best and most widely applied assays for Ras biological activity.