Heterogeneity of estrogen binding sites in mammary tumors.

This paper provides a brief summary of our recent work on the heterogeneity of estradiol binding sites in mammary tumors of the mouse and human. Mammary tumors from both species contain the classical estrogen receptor and a second site(s) which we call type II. These type II sites are found in the cytoplasmic and nuclear compartments of both tumors, and can interfere with the measurement of estrogen receptors. The function of type II sites is not known; however, an assessment of these sites in conjunction with an accurate measurement of the estrogen receptor may provide a clearer picture of the true hormone dependence of breast tumors.