Permanent changes in brain genes may not be so permanent after all

In normal development, all cells turn off genes they don't need, often by attaching a chemical methyl group to the DNA, a process called methylation. Historically, scientists believed methyl groups could only stick to a particular DNA sequence: a cytosine followed by a guanine, called CpG. But in recent years, they have been found on other sequences, and so-called non-CpG methylation has been found in stem cells, and in neurons in the brain.