Estrogen receptor. Unoccupied sites in nuclei of a breast tumor cell line.

Normal estrogen responsive tissues such as the rat uterus contain unoccupied cytoplasmic receptors (Rc) for estrogen (E).After binding estradiol, these receptors are translocated into the nucleus.In contrast to normal target tissues, we find that nuclei of the human breast cancer cell line MCF-7 contain high levels of estrogen receptor (Rn) even when grown in estrogen-free serum.This nuclear localization of Rn is demonstrated by a protamine sulfate assay.Rn is