ESR1 and breast cancer: In this study we aim to address the following questions: (1) whether estrogen suppresses miR-124 expression through ER, and which subunit of ER is required for estrogen-regulated miR-124 downregulation; (2)what is the role of miR-124 in ER positive and ER negative BC cell proliferation, migration and invasion with ER treatment; (3) what is/are functional target(s) of the miR-124; (4) whether the target(s) is/are regulated by estrogen in ER positive breast cancer cells; and (5) the role of miR-124 in regulating ER positive breast tumor growth and angiogenesis.