Illustration of the histone methyltransferase SET8 (green) in complex with the cofactor S-adenosyl-methionine (SAM, red) bound to a nucleosome. Nucleosomes are the fundamental repeating unit used to package DNA (deoxyribonucleic acid, blue) inside cell nuclei. They consist of DNA wrapped around histone proteins. Histone methyltransferases transfer methyl groups to amino acid residues on histones, a process known as methylation. Histone methylation can activate or repress the transcription of genes without changing the actual genetic sequence and so is an example of an epigenetic change.