Abnormal estrogen receptor in clinical breast cancer.

We have discovered a number of estrogen receptor variants in clinical breast cancer tissues. We have base-pair insertions, transitions, and deletions of exons 3, 5 and 7. Using a transactivation assay we have discovered receptors with outlaw function consisting of both dominant-positive receptors which are transcriptionally active in the absence of estrogen, and dominant-negative receptors which are transcriptionally inactive themselves but prevent normal estrogen receptor function.