One of the key breakthroughs in the epigenetics/chromatin field in the last several years was the identification of enzymes capable of removing the methyl group from methylated lysines in histone proteins. Lysine-specific demethylase 1 (LSD1) was the first such enzyme identified, which has been shown to demethylate histone H3 on lysine 4 (H3K4) and lysine 9 (H3K9). LSD1 is essential for mammalian development and likely involved in many biological processes. Recent studies show that LSD1 demethylates p53 and Dnmt1 and regulates their cellular functions, indicating that LSD1 fulfills its biological functions by directly acting on both histone and non-histone proteins. LSD1 contains several defined domains and associates with a number of protein complexes. Interacting partners of LSD1 may play key roles in determining/modulating the activity and specificity of LSD1.