We have used the screening techniques of CMC and SSCP of selected PCR fragments, and also gel retardation assays, to discover a number of ER variants in clinical breast cancer tissues. We have found base pair insertions, transitions, and deletions, and deletions of exons 3, 5 and 7. Using a yeast transactivation assay, we have discovered receptors with outlaw function consisting of both dominant positive receptors that were transcriptionally active in the absence of oestrogen and dominant negative receptors that were transcriptionally inactive themselves but prevented normal ER function. Future efforts should focus in particular on such dominant positive and dominant negative variants. With regard to positive variants, we should like to know whether they stimulate tumour growth and, if so, whether they can be turned off. With regard to dominant negative variants, we should like to determine whether they can inhibit tumour growth and, if so, whether they can be turned on.