5-Hydroxymethylcytosine: A new player in brain disorders?

5-Hydroxymethylcytosine (5hmC), a novel modified cytosine, is oxidized from 5-methylcytosine (5mC) by the ten-eleven translocation (Tet) protein family. The specific distribution of 5hmC in mammalian brain and its roles in gene regulation suggest that 5hmC is important in brain development. 5hmC may also contribute to the mechanisms underlying neurological diseases. Here, we summarize the current knowledge of 5hmC, with an emphasis on its roles in neurodevelopmental and neurodegenerative disorders.