The reproductive hormone, estrogen, contributes to the development of breast cancer by binding to the estrogen receptor (ER) in the nucleus, triggering cell growth and tumor promotion.
During the complex multi-step process of tumor promotion, this common cancer is initiated as hormone-responsive (HR), non-metastatic cancer, followed by a gradual transition into a highly metastatic hormone-insensitive (HI) variety which lacks the functional estrogen receptor.