Scheme showing the structure of cytosine, 5-methylcytosine, and 5-hydroxymethylcytosine. Adding a methyl group to the DNA base cytosine represses gene expression in mammalian cells. To remove the ...

Mutations are changes in the molecular "letters" that make up the DNA code, the blueprint for all living cells. Some of these changes can have little effect, but others can lead to diseases, including ...