ICOSLG, also known as ICOS ligand isoform a, is a number of the B7 family of co-stimulatory molecules related to B7-1 and B7-2. This protein is a transmembrane glycoprotein with extracellular IgV and IgC domains, and binds to ICOS on activated T cells. Its dependent signaling may play an active role in a proliferative response rather than in cytokine and chemokine production. Recombinant human ICOSLG, fused to hIgG-His-tag at C-terminus, was expressed in insect cell and purified by using conventional chromatography techniques.