The inducible costimulator molecule ICOS (Inductible costimulator), also known as AILIM (activation inducible lysophosphate immunomediator motor) or CRP-1 (CD28 related protein-1), is a member of the CD28 family. ICOS is a homologous dimer composed of two subunits, belonging to the type I transmembrane protein and the immunoglobulin superfamily. ICOS is composed of 199aa, containing 20aa of signal peptides, 121aa of extracellular domain, 23aa of transmembrane domain, and 35aa of cytoplasmic domain. Its cytoplasmic region contains the YMFM motif, which can bind to PI3K kinase; The extracellular domain contains a V-like Ig functional region, where the FDPPF motif is a structure that binds to the corresponding ligand ICOSL. The gene product of COSL is a transmembrane glycoprotein composed of 309 amino acids, approximately 63-72 kD. The extracellular domain is similar to other molecules in the B7 family, including the IgV and IgC regions. The intracellular segments of this molecule are spliced to form different isomers according to different physiological environments and cellular states. Among them, four cysteine and tyrosine at position 80 of the IgV region are conserved sites, which are not only markers of the B7 family but also crucial for the formation of immunoglobulin like structures.