 The phenomenon of methylation makes a similar point in a slightly different way. The nucleotides that comprise the familiar sequence are cytosine, thymine, adenine and guanine). When a methyl group attaches to the cytosine molecule the resultant nucleotide is not, strictly speaking, cytosine, but 5-methyl cytosine. So unless one takes the letter ‘C’ in the standard representation of sequence to mean, rather counterintuitively, “cytosine or 5-methyl cytosine”, it is only a partially accurate representation of the feature of the genome it purports to represent. More importantly, it is a representation that fails to capture crucial functional aspects of the genome.