ESR1 and breast cancer: For example, although activation of the FGF signaling in the estrogen receptor-positive (ER+) MCF7 breast cancer cells is unable to enhance cell proliferation [21], activation of FGFR1 in the ER-negative (ER-) human mammary epithelial cells increases cell proliferation, and knockdown of FGFR1 in the ER- mouse breast cancer cells also inhibits cell proliferation [22, 23].