KDM1B is a histone demethylase that demethylates lysine 4 of histone H3, a specific tag for epigenetic transcriptional activation, thereby acting as a corepressor. KDM1B is able to demethylate both mono- and di-methylated lysine 4 of histone H3, but has no effect on tri-methylated lysine 4, mono-, di- or tri-methylated lysine 9, mono-, di- or tri-methylated lysine 27, mono-, di- or tri-methylated lysine 36 of histone H3, or on mono-, di- or tri-methylated lysine 20 of histone H4. KDM1B is also required for de novo DNA methylation of a subset of imprinted genes during oogenesis. It acts by oxidizing the substrate by FAD to generate the corresponding imine that is subsequently hydrolyzed.