Research Summary 5-methylcytosine is a minor base in mammalian DNA, which plays a disproportionately critical role in development. DNA methylation is very dynamic during embryogenesis and is vital for parental imprinting, X inactivation, silencing of endogenous retroviruses as well as the regulation of genomic stability. De novo methylation and demethylation also occur in somatic cells during differentiation, tumorigenesis and aging. In contrast to the established relation between DNA methylation, DNA methyltransferases and gene silencing, the enzymes and processes regulating DNA demethylation are not well understood. We have recently discovered that TET1 catalyzes the conversion of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (hmC). We also found that hmC can be detected in the genome of mouse ES cells, and that both TET1 levels and hmC levels decline when ES cells are differentiated. These results suggest that hmC is a normal constituent of mammalian DNA, and identify TET1 as an enzyme with a potential role in epigenetic regulation through modification of 5mC. This study described a new class of enzymes that catalyze a new modification of DNA and alters our perception of how DNA methylation status may be regulated in cells. The description of the regulated conversion of 5mC to hmC raises an enormous number of new questions that are of pressing importance. The goal of my laboratory is to integrate hmC into known pathways of 5mC metabolism and to determine how hmC exerts its influence on the genome with the ultimate goal of understanding the role that 5mC and hmC play in genomic stability.