5-hydroxy methyl cytosine (5hmC) is a modification identified in vertebrates 
several decades ago. More recently, a possible role of 5hmC as an epigenetic 
modifier and/or transcriptional regulator has started to emerge, with altered 
levels in early embryonic development, embryonic stem (ES) cell differentiation 
and tumours (Tahiliani et al, 2009; Yang et al, 2012). The balance between 5hmC 
and 5-methyl cytosine (5mC) at gene promoters and CpG islands in the genome 
appears to be linked to pluripotency and lineage commitment of a cell (Ito et 
al, 2010). However, proteins with 5hmC binding capability have not yet been 
identified, and it has been proposed that 5hmC may only be a reaction 
intermediate in the process of demethylation (He et al, 2011; Ito et al, 2011). 
Over the last few years, ten-eleven translocation (Tet) family proteins have 
been shown to be responsible for the conversion of 5mC to 5hmC (Iyer et al, 
2009; Loenarz and Schofield, 2009; Tahiliani et al, 2009). However, how Tet 
family proteins and 5hmC are linked to transcriptional regulation is currently 
not clear.