Logic at the cell surface A major challenge in medical interventions is to target only diseased cells. Although there are biomarkers characteristic of certain cancers, for example, it is unlikely that a single marker can specify a particular cell type. Lajoie et al. addressed this problem by designing protein switches called Co-LOCKR that bind to antigens on the cell surface and activate through a conformational change only when there is a precise combination of antigens. They designed switches that can perform AND, OR, and NOT logic. On the path toward applying this technology, they used Co-LOCKR to direct chimeric antigen receptor T cells to tumor cells expressing specific antigens. Science , this issue p. 1637