Regulation of transcription at the chromatin level. Transcriptionally active chromatin is characterized by a low level of organization. This state has active histone marks, such as H3K4me2/3 and H3K9ac, which are introduced by the Trithorax group proteins (TrxGs) and histone acetyltransferases (HACs), respectively. Transcriptionally silent chromatin undergoes compaction by the mechanisms that are mediated by HP1 and linker histone H1. Core histones have the inactive chromatin marks H3K9me2/3, H3K27me2/3, and H2AK119ub, which are introduced by lysine histone methyltransferases (HMTs), PRC2, and PRC1, respectively.