Histone methylation, molecular model - stock illustration

Molecular model of the disruptor of telomeric silencing 1-like (DOT1L, mid blue) protein complexed with a histone (dark blue). Histones are proteins that DNA (deoxyribonucleic acid) wraps around before being packaged into chromosomes. DOT1L transfers a methyl group to a lysine amino acid residue on the histone, a process known as methylation. This transfer can only occur if the protein ubiquitin (light blue) has already bound to the histone. 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