second, cancer cells can have mutated cell-surface receptors for mitogens. the protein kinase domain found on mitogenic receptors is often hyperactivated in cancer cells, remaining turned on even in the absence of external mitogens. additionally, some cancers are associated with an overproduction of mitogenic receptors on the cell surface. with this mutation, cells are stimulated to divide by abnormally low levels of mitogens. one such example is her2, a receptor tyrosine kinase that responds to the mitogen egf. overexpression of her2 is common in 15-30% of breast cancers, allowing the cell cycle to progress even with extremely low concentrations of egf. the overexpression of kinase activity in these cells aids in their proliferation. these are known as hormone-dependent breast cancers, as the kinase activation in these cancers is connected to exposure to both growth factors and estradiol.