ICOS: ProductsInducible costimulator (ICOS), also called AILIM (activation-inducible lymphocyte immunomediatory molecule) and CRP-1 (CD28-related protein-1), is a member of the growing CD28 family of immune costimulatory receptors. Other family members are CD28, CTLA-4 and PD-1. Human and mouse ICOS share approximately 72% amino acid sequence identity. ICOS is expressed on most CD45RO+ cells. B7-H2, a member of the B7 family of costimulatory ligands, has been identified as the ICOS ligand.