Most breast cancer cells  (especially in postmenopausal aged women) will still be sensitive to the effects of estrogen or progesterone, which can be measured by the pathologist and reported as having positive estrogen or progesterone receptors.

A common misconception among women is that this somehow 'proves' that hormones caused their cancer. In truth a positive hormone receptor test is a good thing for two reasons: better prognosis and sensitivity to hormone therapy.

Better prognosis: since all cancer start with a normal cell that goes bad... the closer the cell looks like a normal cell (well differentiated) or acts like a normal cell (a normal breast cell responds to hormones. e.g. in pregnancy the breast get larger)... the less serious the cancer (the less mutated or dangerous.) So the prognosis for women with estrogen receptor cancer is much better than women who have receptor negative cancer. Compare the survival by tumor size and number of nodes in early cases and you will see that survival is much better for hormone + cases...go here. Also in cases of advanced, metastatic breast cancer, the survival is longer in cases that are hormone +...go here. There are estrogen receptors (ERP) and progesterone (PgR) and best survival is if both are positive (sensitive) and worst if both are negative (go here).

Sensitivity to hormone therapy. If the cancer shows positive hormone receptors then the cancer is quite likely to respond to treatment with hormone therapy drugs (e.g. Tamoxifen or Arimidex, see hormone therapy.)