PROJECT SUMMARY/ABSTRACT My lab seeks to define the molecular logic of complex cell behaviors— how cells go from sets of interacting molecules to the emergent properties of living systems. We currently focus on three questions: how neutrophils control their shape and movement, how lymphocytes detect rare foreign peptides in a sea of self- peptides, and how mouse embryonic stem cells regulate transcriptional activation. Studying a diversity of cell types and behaviors makes it easier to identify the general principles of cellular decision-making beyond the particulars of a given system. And it opens up more interfaces for cross pollination between different projects in the lab. Transformative science often happens at interfaces, so we seek to borrow tools and concepts from other fields to address open questions in cell biology and frequently develop new tools when they are needed to accelerate progress. For instance, optogenetics enables us to plug into defined signaling nodes and test the logic of subcircuits in a manner that circumvents the feedback, redundancy, and compensation that confound investigation with standard approaches. Our most common strategy is to pair biosensors for visualizing the quantitative dynamics of these processes in living cells with precision tools to control the regulators of these behaviors. For cell migration, we are breaking down the complex process of directed movement into its fundamental pieces—how a cell decides to initiate a protrusion, how the shape of the protrusion is specified, how the protrusions compete with one another to enable cell polarity, and how this process is biased by external gradients. For T cell activation, we are investigating how cells convert small differences in antigen binding to large changes in cell activation by leveraging a light-responsive T cell antigen. Finally, we are investigating the logic of transcriptional activation—in particular how enhancers activate promoters and how transcription factor dynamics specifies the pattern of gene activation.