Because I am not personally ER-/PR+, I mentally filed the information as an interesting note but didn't save articles about it, so today I went on safari on the net to see if I could answer your question (and save face!) My recall is that I saw it mentioned more than once as a possible reason why that group is so small.
How small is it? Well, in one recent study of 1,153 cases the percentage that were ER-/PR+ was 7%. In another study, only one of 942 patients had an ER–/PR+ tumor (0.1%).
(A side note: As a hunter on safari, perhaps one should always take pictures of interesting animals even if one doesn't plan to shoot them. I kept looking.)
I think the following could possibly have been the basis for the in-passing memory that I have:
"A small number of breast cancers are found to be ER negative, PR positive; some of these respond to endocrine therapy. This observation is difficult to reconcile with the accepted view that PR synthesis depends on estrogen action. However, it has been reported that many such tumors, especially in premenopausal patients, actually do contain estrogen receptors that are not detected by steroid-binding assay unless the cytosol is first treated with charcoal."
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?...d.section.12459
Book titled: Cancer Medicine, 6th ed.
Threaded Mode