HDAC9 and neoplasm: Some studies have reported HDAC inhibitors’ potential in enhancing the efficacy of immunotherapy and even reverse the insensitivity to ICI (Ugurel et al., 2019) by enhancing the anti-tumor activity of CD8+ T cells (Que et al., 2021) and NK cells (Hicks et al., 2018; Kim et al., 2020), reducing the number of M2 macrophages (Knox et al., 2019; Kim et al., 2020), impairing the immunosuppressive function of MDSCs (Kim et al., 2020; Que et al., 2021) and increasing the expression of PD-L1 on tumor cells (Hicks et al., 2018; Que et al., 2021).