Some cells make a different class of growth factor receptor called the Epidermal Growth Factor Receptor. There is a growing body of experimental evidence showing that breast cancer cells that make Epidermal Growth Factor Receptors are more resistant to hormonal therapy and have a poorer prognosis. Several investigators have found that the Epidermal Growth Factor Receptor can activate the estrogen receptor, even in the presence of estrogen-blocking drugs. Growth of these cells can be slowed by blockade of both Epidermal Growth Factor Receptor signaling and estrogen-receptor signaling. Lapatinib is a small molecule which can inhibit two different forms of the Epidermal Growth Factor Receptor. It has been studied in people with a number of different cancers, including breast cancer, and a safe dose and its common side effects have been defined.