Gene activation and repression is specifically regulated by histone methylation status at distinct lysine residues. Lysine specific demethylase 1 (KDM1/LSD1) is a long-sought histone demethylase that specifically demethylates mono and di methyl histone H3 at K4 and K9. Thus KDM1 is a specific tag for epigenetic transcriptional activation, thereby acting as a corepressor.