Name % identity to CD28 Ligand binding motif Expression cell type CD28 100% MYPPPY T CTLA-4 30% MYPPPY T ICOS 24% FDPPPF T, NK PD-1 21% ? T, B, MΦ Abbreviations: T = T cells; B = B cells; NK = natural killer cells; MΦ = macrophages

Table 1: CD28 family receptors.