Cluster of Differentiation 80, also called B7-1, is a member of cell surface immunoglobulin superfamily which plays key, yet distinct roles in the activation of T cells. Mouse CD80 consists of an extracellular domain (ECD) with twoimmunoglobulin-like domains, transmembrane segment, and cytoplasmic domain. B7-1/CD80 and B7-2/CD86, together with their receptors CD28 and CTLA4, constitute one of the dominant co-stimulatory pathways that regulate T- and B- cell responses. CD80 is mostly expressed on the surface of antigen-presenting cells including activated B cells, macrophages and dendritic cells. 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.