It is fairly well accepted that the presence of estrogen receptor (ER) identifies those breast cancer patients with a lower risk of relapse and better overall survival [Clark and McGuire, 19881, and the measurement of ER has become a standard assay in the clinical management of breast cancer. Receptor status also provides a guideline for those tumors which may be responsive to hormonal intervention [McGuire 1978; Osborne et al., 1980; Rose et al., 19851. But only about half of ER-positive patients will respond to the various hormonal therapies available, and of those who do initially respond, most will eventually develop hormonally unresponsive disease following a period of treatment even though ER is often still present. Loss of ER from initially ER-positive tumors biopsied again at a later date has been estimated at only 19% [Gross et al., 19841. Obviously the simple measurement of ER presence by ligand-binding assays does not provide us with an adequate estimate of the functional state of the receptor.
In 1985 Sluyser and Mester hypothesized that the loss of hormone dependence of certain breast tumors may be due to the presence of mutated or truncated steroid receptors that activate transcription even in the absence of hormone [Sluyser and Mester, 19851. Based on the recent identification of several ER sequence variants in human breast cancer cell lines and tumor specimens, we would now like to propose that some of these identified mutations play a role in receptor dysfunction in vivo, and will review those ER mutations which may prove to be important in breast cancer progression. o 1993 Wiley-Liss, Inc.