HDAC9 and breast cancer: In liver cancer, breast cancer, bladder cancer, renal clear cell cancer, and colorectal cancer, HDAC inhibitors have also been reported to upregulate the levels of tumour antigen/costimulatory molecules/receptors (such as major histocompatibility complex), activate NK cells or cytotoxic T cells, and relieve the inhibition of immunosuppressive cells (such as regulatory T cells and myeloid-derived suppressor cells).26