HDACi have been tested in all breast cancer subtypes because preclinical research shows that this class of drug can target breast cancer in a variety of ways, including relief of transcriptional repression with an impact on the epithelial-mesenchymal transition (EMT), reactivation of silenced oestrogen receptor (ER) in hormone receptor-negative tumors, restoring the sensitivity of hormonal therapy in estrogen-positive tumors, and modulation of transcriptional repression with an impact on the epi (Connolly et al., 2017). Here, ESR1 is linked to breast cancer.