ESR1 and breast carcinoma: To better characterize HDAC9 effect on estrogen signaling in breast cancer cells, two ERα‐positive MCF7 cell clones that stably express HDAC9FL (MCF7‐HDAC9FL, clones 4‐1 and 1‐1) were selected and characterized relative to control clones (P1 and P2; empty vector).