Inhibition of estrogen receptor action by a naturally occurring variant in human breast tumors.

It is fairly well accepted that the presence of estrogen receptor (ER) and progesterone receptor (PgR) identifies breast cancer patients with a lower risk of relapse and better overall survival. But patients with discordant receptors, the ER+/PgR- phenotype, are often intermediate in clinical response. We focused upon this group of patients and have identified a truncated ER which is abundant in some ER+/PgR- breast tumors and which inhibits the binding of wild-type ER to its cognate response element. This variant interferes in a dominant negative manner with wild-type ER function and may represent a mechanism for modulation of estrogen responsiveness.