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DNA and methyltransferase complex. Molecular model showing a molecule of methyltransferase bound to a DNA (deoxyribonucleic acid) strand (red and yellow). Methyltransferase, or methylase, enzymes transfer a methyl group from a donor to an acceptor. This methylation often occurs on nucleic bases in DNA or amino acids in protein structures. DNA methylation is often used to silence and regulate genes without changing the original DNA sequence (KEYSTONE/SCIENCE PHOTO LIBRARY/LAGUNA DESIGN)