RPS6KB1 and breast carcinoma: Furthermore, histone deacetylase 1 (HDAC1), an essential epigenetic regulator that modulates chromatin structures, has also been identified as a substrate of S6K1 upon mitogen stimuli in breast cancer cells, which accounts for HDAC1 hyperphosphorylation and activation, leading to deacetylase-dependent suppression of ERα transcription.