PD-1, a member of the CD28/CTLA-4/ICOS costimulatory receptor family, delivers negative signals that have profound effects on T and B cell immunity. The 2.0 A crystal structure of the extracellular domain of murine PD-1 reveals an Ig V-type topology with overall similarity to the CTLA-4 monomer; however, there are notable differences in regions relevant to function. Our structural and biophysical data show that PD-1 is monomeric both in solution as well as on cell surface, in contrast to CTLA-4 and other family members that are all disulfide-linked homodimers. Furthermore, our structure-based mutagenesis studies identify the ligand binding surface of PD-1, which displays significant differences compared to those present in the other members of the family.