Abstract Lysine Specific Demethylase 1 (LSD1/KDM1A) plays an important role in the regulation of histone methylation at lysine residues, and is currently being validated as an attractive therapeutic target for many diseases, particularly for multiple forms of cancer. Methylation of lysine residues on histones can signal transcriptional activation or repression depending on the specific residue involved. H3K4me2 is a transcription-activating chromatin mark at gene promoters, and demethylation of this mark by LSD1 is thought to prevent expression of tumor suppressor genes important in human cancer. In contrast, methylation of H3K9 is a repressive mark and LSD1 activity has been shown to upregulate tumor promoting pathways. Thus, LSD1 is emerging as an important target for the development of specific inhibitors as a new class of antitumor drugs. Several LSD1 inhibitors have been reported, but they have shown poor selectivity and/or pharmacological properties, making further exploration of LSD1 biology difficult. We previously reported the identification of CIT-0665, an LSD1 inhibitor identified through a virtual screening effort in our lab. Evaluation of the structure activity relationships of multiple analogs of our LSD1 inhibitor led to the identification of HCI-2528, which is more potent and exhibits improved drug like properties. Using a cell viability assay, a large panel of cancer cell lines was tested for sensitivity to HCI-2528. In addition to the well-documented sensitivity of breast cancer cell lines, we also found that Ewing's sarcoma cell lines are uniquely sensitive to LSD-1 inhibition. We are currently engaged in a bioinformatic effort to determine possible mechanisms of sensitivity from our studies, and performing xenograft studies in mice. In conclusion, HCI-2528 is a novel LSD1 inhibitor with activity in biochemical and cell-based assays. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B84.