Here, down-regulation of HDAC5 by siRNA clearly decreased the expression of Sp1 (24 h post-treatment) and survivin (48 h post-treatment) in the tested ER+ breast cancer cells, indicating that HDAC5 positively regulates the expression of Sp1 and survivin, and suggesting that HDAC5 may promote the induction of hormone therapy resistance in MCF7-TamC3 cells, in part through alteration of the miR-125a-5p-Sp1-survivin signaling pathway (Figure 6I and Supplementary Figure S4B). This evidence concerns the gene SP1 and breast cancer.