CTLA4, also known as cytotoxic T-lymphocyte-associated protein 4, is a co-inhibitory molecule expressed on T cells that mediates the inhibition of T-cell function. This protein is a crucial immune regulator that mediates both negative costimulation signals to T cells, and regulatory T (Treg)-cell extrinsic control of effector responses. It has been implicated in several autoimmune diseases, such as systemic lupus erythematosus. Recombinant human CTLA4, fused to His-tag at C-terminus, was expressed in insect cell and purified by using conventional chromatography techniques.