In recent years, the incidence of breast cancer has been increasing worldwide, and breast cancer is becoming a serious object of public concern. The onset of breast cancer is closely related to the sex hormone estrogen, and estrogen antagonists such as tamoxifen have been used as anti-breast cancer drugs.
During pregnancy, the elevated blood estrogen level induces the proliferation of mammary epithelial cells, leading to the development of the mammary gland in preparation for lactation. The mammary epithelial cells eventually stop proliferation at late stages of pregnancy, impairment of which potentially results in breast tumorigenesis . However, the regulatory mechanisms of mammary epithelial cell proliferation during pregnancy have been unclear.