Unlike normal breast cells, cancer cells arising in the breast do not always have receptors for estrogen.
Breast cancers that DO have estrogen receptors are said to be “estrogen receptor-positive,” while those breast cancers that DO NOT possess estrogen receptors are “estrogen receptor-negative.” In women with estrogen receptor-positive cancers, cancer cell growth is under the control of estrogen. Therefore, such cancers are often susceptible to treatment with tamoxifen (Nolvadex®), because tamoxifen works by blocking the interaction between estrogen and the estrogen receptor.
In contrast, the growth of estrogen receptor-negative cancer cells is not governed by estrogen and is not treated with tamoxifen.