Stanley Cohen purified a protein the Epidermal Growth Factor (EGF), able to induce epidermis proliferation and to elicit precocious eye disclosure and teeth eruption, establishing the "inverse" relationships between the proliferation and differentiation.
Reported variability in HER1 tumor expression and response to HER1-targeted agents, a wide range of EGF concentration in healthy women breasts fluid and the skin rash/tumor response relation to HER1-targeted drugs are discussed as possible examples of individual differences in tissue dependency on HER1 interactions with ligands in normal and cancer tissue.