Additionally, there are many targeted small molecule inhibitors under clinical trials for the treatment of breast cancer, such as the CDK4/6 inhibitor palbociclib (Song et al., 2019; Gan et al., 2020; Ramos et al., 2020), the PI3K/Akt/mTOR pathway inhibitor pictilisib (Altine et al., 2019; Han et al., 2019), the HDAC inhibitor CUDC-101 (Meng et al., 2013). Here, HDAC9 is linked to breast cancer.