Explain the purpose of tamoxifen in breast cancer treatment.

hormone therapy that blocks the effects of estrogen.....

Tamoxifen binds to estrogen receptors and inhibits estrogen-mediated tumor growth. Breast cancer cells that are well-differentiated and ER+ (estrogen receptor positive) will regress under tamoxifen, whereas the drug is ineffective against poorly-differentiated ER- tumors. Biopsy specimens are routinely tested for ER status to inform therapy. Recent studies have also shown benefit to patients taking tamoxifen for "pre-invasive" conditions such as DCIS (ductal carcinoma in situ) which are often the best-differentiated and more ER sensitive tumor cells of all.