methylation of cytosine
" The methylation of cytosine is like a traffic light in that its absence allows the gene to be on, and its presence forces the gene to be off. We need 5-methylcytosines in our genome, but we need them in the right places in the right cells. A few methyl groups are misplaced and a healthy cell that knew its place in the body becomes a cancerous cell that can overrun the body. Four atoms can take a life."
A good essayist she is in the link.
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