Inducible T-cell costimulator (also called CD278) is an immune checkpoint protein that in humans is encoded by the ICOS (Inducible T-cell COStimulator) gene. The protein belongs to the CD28 and CTLA-4 cell-surface receptor family. These are proteins expressed on the surface of immune cells that mediate signalling between them. A surface protein, the ligand, binds specifically to its receptor on another cell, leading to a signalling cascade in that cell. ICOS is a receptor protein expressed on the surface of activated T cells. Its ligand ICOS-L (previously called B7RP-1) is constitutively expressed on B cells. Stimulation of the ICOS receptor on T cells by ICOS-L on B cells is required for the development of follicular helper T (Tfh) cells. ICOS forms homodimers and plays an important role in cell-cell signaling, immune responses and regulation of cell proliferation.