Molecular model showing three of the main epigenetic modifications of DNA (deoxyribonucleic acid, orange) and histones (dark blue). Histones are proteins that DNA wraps around before being packaged into chromosomes. At right two ubiquitin molecules (light blue) are bound to the histone. At upper left the disruptor of telomeric silencing 1-like (DOT1L, mid blue) protein is complexed with a histone. 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 ubiquitin has already bound to the histone. At centre DNA methyl transferase-1 (DMNT1, blue) is complexed with DNA. DMNT1 transfers a methyl group from S-adenosyl methionine (red) to cytosine DNA bases. Histone and DNA methylation can silence and regulate genes without changing the actual genetic sequence and so are examples of epigenetic changes.