B7-1 and B7-2 are glycoproteins expressed on antigen presenting cells. The binding of these molecules to the T-cell homodimers, CD28 and CTLA-4, generate 'costimulatory' and inhibitory signals in T cells, respectively. The crystal structure of the extracellular region of B7-1 (sB7-1), solved to 3 Angstrom resolution, consists of a novel combination of two Ig-like domains, one characteristic of adhesion molecules and the other previously seen only in antigen receptors. In the crystal lattice, ...