Small molecules that can act as chemical modulators of enzyme function are valuable tools for probing protein function and for use as therapeutic agents. Most methyltransferases use the methyl donor ? S-adenosyl-L-methionine (AdoMet) ? as a cofactor. Current methyltransferase inhibitors display limited specificity, indiscriminately targeting all enzymes that use AdoMet. We have designed and implemented a high-throughput screen for the identification of small molecules that specifically inhibit protein arginine N-methyltransferase (PRMT) activity. Using this approach we have identified a lead compound, AMI-1, that inhibits PRMT activity but not lysine methyltransferases. Thus, unlike current methyltransferase inhibitors, which display limited specificity, we have demonstrated the feasibility of identifying a new generation of compounds that will discriminate not only between protein methyltranferases, but also be selective for DNA, RNA, lipid and polysaccharide methyltransferases. We will use knockout mouse models to confirm that PRMT substrates identified though in vitro screens are indeed in vivo targets of these enzymes. Small molecule inhibitors will be used to phenocopy genetic models and probe the biological roles of PRMTs in systems where these enzymes are required for survival.