HDAC9 and breast cancer: Secondly, histone H1 is closely associated with BC development and progression, with common modifications such as acetylation, phosphorylation, methylation, ubiquitination, SUMOlyation, and ADP-ribosylation, where the histone acetylation state is regulated by histone acetyltransferase (HAT) and histone deacetylase (HDAC) (Portela and Esteller, 2010; Telu et al., 2013; Shen et al., 2015).