Although increasing evidence has shown that pan‐HDAC inhibitors such as vorinostat and SB939 can reduce the occurrence of BCBM, it is still unclear which specific HDAC protein plays a key role in the process of breast cancer cell metastasis to the brain and whether such effect is directly caused by a particular HDAC protein or derived from HDAC downstream gene targets. Here, HDAC9 is linked to breast cancer.