LY9, as known as T-lymphocyte surface antigen Ly-9 isoform, is a type 1 transmembrane glycoprotein in the SLAM subgroup of the CD2 family. This protein consists of an extracellular domain with two lg-like V-set and two lg-like truncated C2-set domains. It is expressed on T and B cells, thymocytes, and more weakly on NK cells. Homophilic binding between LY9 molecules is mediated by the N-terminal lg-like domain. Also, this protein knockout mice show minimally impaired immune responses, suggesting functional redundancy with other molecules. Recombinant human LY9, fused to His-tag at C-terminus, was expressed in insect cell and purified by using conventional chromatography techniques.