Estrogen alters levels of breast-cancer stem cells.

There is good evidence that, at least in some cancer types, not all cancer cells are equally able to divide and form tumors.  The select cells that acts as the driving force in the development of cancer are known as cancer stem cells (CSC).  Evidence suggests CSC are responsible for the development of breast cancer.  In a recent study, researchers showed that estrogen reduces the number of CSC in normal and cancerous breast tissue.  The results may partially explain why cancers that are estrogen receptor positive tend to have better prognoses.