This Program proposes an overarching hypothesis that rational targeting of ?-secretase, a multi-subunit intramembrane cleaving protease that has several biologically important substrates, may be considered as a therapeutic target for a number of diseases. The three projects in this Program address the use of ?-secretase inhibitors in breast cancer (Project 1), and graft-versus-host disease (Project 2), and propose biochemical strategies to enable us to better understand the mechanism of action of ?-secretase (Project 3). These projects are integrated with a goal of better understanding the mechanism of action of ?-secretase inhibition in normal, as well as, disease settings. The Principal Investigators of this Program have worked together, shared data and published as a group for the past six years. The Program is significantly enhanced by a Chemistry Core that has the proven ability to synthesize a wide variety of ?-secretase inhibitors that allow members of the Program to explore the biological properties of these inhibitors in an environment free from commercial restriction. Such an approach will provide a scientific basis for optimal GSI selection matched to a particular disease.