B7-1 also known as protein CD80 is a molecule found on activated B cells and monocytes which provides a costimulatory signal necessary for T cell activation and survival.

The cDNA for B7-1 predicts a type I membrane protein. The protein is predicted to contain two extracellular domains structurally similar to those of Ig, a hydrophobic transmembrane region, and a short cytoplasmic domain.

The CD80 and CD86 genes encode B7-1 and B7-2, respectively, which are structurally similar members of the immunoglobulin superfamily expressed on a variety of hematopoietic cell types. B7-1 and B7-2 provide a costimulatory signal to T cells by interacting with CD28 and CTLA4.