In summary, although TGF-alpha was initially found in tumors, a number of later studies, some of them from the author's laboratory, have shown that TGF-alpha should no longer be considered a tumor associated growth factor. Rather, TGF-alpha is a normal physiological ligand for the EGF receptor. Table 2 lists some of the normal cellular sources of TGF-alpha. Our list is incomplete, but we know that TGF-alpha is made in keratinocytes and a number of epithelial cells, including gut and breast epithelial cells. It seems very likely that TGF-alpha is a major growth factor secreted by cells of epithelial origin. Zena Werb's and Russell Ross's groups have shown that activated macrophages make TGF-alpha. We have shown that brain makes TGF-alpha and Jeff Kudlow has found TGF-alpha made in the pituitary. Data from several sources, including David Lee, the author's laboratory, and Zena Werb's laboratory has shown that TGF-alpha is made during embryonic development. Therefore, it is now important to look at TGF-alpha in its normal physiological context. Finally, it should be stressed that, as was mentioned above, TGF-alpha is not necessarily a secreted growth factor 50 amino acids long. There is quite a bit of processing of the larger precursor that may or may not take place. This processing, which determines the ultimate size and location of the molecule, is also likely to influence its physiological action.