Fto is a dioxygenase that repairs alkylated DNA and RNA by oxidative demethylation. Fto has highest activity towards single-stranded RNA containing 3-methyluracil, followed by single-stranded DNA containing 3-methylthymine. Fto has low demethylase activity towards single-stranded DNA containing 1-methyladenine or 3-methylcytosine. Fto has no activity towards 1-methylguanine and no detectable activity towards double-stranded DNA. Fto requires molecular oxygen, alpha-ketoglutarate and iron. Fto contributes to the regulation of the global metabolic rate, energy expenditure and energy homeostasis. Fto contributes to the regulation of body size and body fat accumulation.