B7-1 and B7-2, together with their receptors CD28 and CTLA-4, constitute one of the dominant co‑stimulatory pathways that regulate T‑ and B‑cell responses. Although both CTLA-4 and CD28 can bind to the same ligands, CTLA-4 binds to B7-1 and B7‑2 with a 20‑100 fold higher affinity than CD28 and is involved in the down‑regulation of the immune response. B7-1 is expressed on activated B cells, activated T cells, and macrophages. B7‑2 is constitutively expressed on interdigitating dendritic cells, Langerhans cells, peripheral blood dendritic cells, memory B cells, and germinal center B cells. Additionally, B7-2 is expressed at low levels on monocytes and can be up‑regulated through interferon gamma. B7-1 and B7-2 are both members of the immunoglobulin superfamily. Mouse B7-2 is a 309 amino acid (aa) protein containing a putative 23 aa signal peptide, a 221 aa extracellular domain, a 21 aa transmembrane domain, and a 44 aa cytoplasmic domain. Mouse B7-2 and B7-1 share 28% amino acid identity. Mouse and human B7-2 share 50% amino acid identity. However, it has been observed that both human and mouse B7‑1 and B7‑2 can bind to either human or mouse CD28 and CTLA‑4, suggesting that there are conserved amino acids which form the B7-1/B7-2/CD28/CTLA-4 critical binding sites.