Abstract: The choice of developing T cells to adopt the CD4 helper or CD8 killer cell fate is a valuable model for understanding mammalian cell fate decisions and is a key step in shaping the adaptive immune response. While it is known that TCR recognition of self pMHC ligands in the thymus controls cell fate, the molecular links between TCR signaling and the activation of the lineage-defining transcriptional networks remains unknown. Guided by a high-resolution single cell map of T cell development, we propose to define the molecular links between different branches of the TCR signaling pathway and the transcriptional network that specifies the CD4 fate (Aim 1). We will also probe the factors that determine why thymocytes that recognize MHC-2 fully activate this network, whereas thymocytes that recognize MHC-1 do not (Aim 2). The proposed studies will reveal fundamental principles underlying mammalian cell fate decisions and help to understand the molecular mechanisms that link the killer versus helper T cell effector functional programs to the recognition of MHC 1 versus 2.