The genome is tightly condensed into chromatin, which needs to be loosened for transcription to occur. In order to halt the transcription of a gene the DNA must be wound tighter. This can be done by modifying histones at certain sites by methylation. Histone methyltransferases are enzymes which transfer methyl groups from S-Adenosyl methionine (SAM) onto the lysine or arginine residues of the H3 and H4 histones. There are instances of the core globular domains of histones being methylated as well.