Illustration of a SAM(s-adenosyl methionine)-dependent methyltransferase ribozyme. Ribozymes are RNA (ribonucleic acid) molecules that catalyse certain biochemical reactions. Methyltransferase ribozymes transfer methyl groups to RNA nucleotides, a process known as methylation. RNA methylation can activate or repress the transcription of genes without changing the actual genetic sequence and so is an example of an epigenetic change.