ESR1 and breast cancer: Down-regulation of HDAC2 by siRNA further decreased the viability of ZR-75-1 cells cultured under estrogen-deprived conditions (i.e., reduced by 52% in estrogen-deprived medium vs. 29% in full medium), suggesting HDAC2 may exhibit an enhanced pro-cell survival role in ER+ breast cancer cells experiencing estrogen-deprived stress (Figure 2C).