LSD1 is a histone demethylase whose actions on specific lysine residues alter transcription of chromosomal DNA. It also inhibits the tumor suppressor activity of p53 by demethylating a specific lysine residue. Inhibitors of LSD1 are important tools used to elucidate mechanisms of transcription and cell cycle progression and have therapeutic potential for treating cancer. Cayman’s LSD1 Inhibitor Screening Assay provides a convenient fluorescence-based method for screening LSD1-specific inhibitors. The assay is based on the multistep enzymatic reaction in which LSD1 first produces H2O2 during the demethylation of lysine 4 on a peptide corresponding to the first 21 amino acids of the N-terminal tail of histone H3. In the presence of horseradish peroxidase, H2O2 reacts with ADHP to produce the highly fluorescent compound resorufin that can be analyzed with an excitation wavelength between 530-540 nm and an emission wavelength between 585-595 nm.