Histone modifications mediate changes in gene expression by altering chromatin structure or by serving as a platform to recruit other proteins. LSD1 is a recently discovered amine oxidase that catalyzes the lysine-specific demethylation of histone proteins via an FAD-dependent oxidative reaction (1). Methylation on histone H3-K9 is thought to play an important role in heterochromatin formation, while methylation on arginine and some lysine residues (such as H3-K4) is associated with active transcription (2). LSD1 associates with various proteins, including HDAC1/2, CoREST, and BHC80, that act to regulate LSD1 activity in vivo, and in a histone H3-K4-specific methylase complex that is involved in transcriptional regulation (3,4). Experiments have shown that CoREST, a SANT domain-containing corepressor (5) acts to enhance LSD1 activity, while BHC80, a PHD domain-containing protein (6), inhibits CoREST/LSD1 activity in vitro (3). LSD1-mediated histone demethylation thus may have significant effects on gene expression.