HDAC9 and breast cancer: Mechanisms underlying endocrine therapy resistance in ER + breast cancer include long non-coding RNAs (lncRNAs) Lnc-DC (Peng et al., 2021), crosstalk with conditional macrophages (Castellaro et al., 2019), upregulation of IL-6 and IL-6R by interfering with the inhibitory activity of NCOR2 (Tsoi et al., 2021), HOXB13 transcriptionally upregulating IL-6 (Shah et al., 2013), or loss of HDAC ZIP (Zhu et al., 2020), which mediates STAT3 activation or signaling enhancement, upregulates anti-apoptotic signals, or activates alternative survival pathways.