One of the central mysteries of immunology is self-tolerance. How does the human body select ~10e12 T lymphocytes, that are reactive to foreign pathogens but tolerant to normal cellular constituents of the host? Over the last few years, my laboratory identified 2 fundamental mechanisms used by thymocytes to establish T cell tolerance. We demonstrated that the affinity threshold for negative selection is a constant for all thymocytes expressing MHC I restricted TCRs. This binding affinity threshold (KD H 6 ¼M; estimated T1/2 H 2 sec) is the fundamental biophysical parameter used by TCRs to delete autoimmune T cells. We also established how the TCR generates distinct signals for positive and negative selection. At the selection threshold, a small increase in ligand affinity for the T-cell antigen receptor leads to a marked change in the activation and subcellular localization of Ras and mitogen-activated protein kinase (MAPK) signaling intermediates. The ability to compartmentalize signaling molecules differentially within the cell endows the thymocyte with the ability to convert a small change in analogue input (affinity) into a digital output (positive versus negative selection) and provides the molecular basis for central tolerance. In the present application, we plan to fully understand 1-how the biophysical events during antigen binding to the TCR initiate an intracellular signal; 2-how these signals program an unambiguous cell fate and 3-how the system fails, when an autoimmune T cell is generated and activated. We will use a combination of transgenic and knockout mice, biochemistry and molecular imaging to fully define how the TCR functions as a molecular switch.