The DNA base cytosine has a tendency to play dress-up, gaining and shedding chemical modifications. For more than 40 years, scientists have known that methyl groups attached to cytosine's fifth carbon atom can alter gene expression. These epige-netically marked bases, called 5-methyl-cytosines (5mCs), help to determine how hundreds of cell types in the human body differentiate and maintain their identities, despite having the same genetic backgrounds. Recently, researchers have rediscovered a mostly ignored epigenetic variant that results when a methyl group on a cytosine takes on a hydroxyl group to form 5-hydroxymethylcytosine (5hmC). The favored method for detecting meth-ylation is bisulfite sequencing, which converts unmodified cytosine to ura-cil, which then reads as thymine following PCR amplification. Modified cyto-sines continue to read as cytosines.
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