CD4 is a member of the immunoglobulin superfamily and is implicated as associative recognition element in MHC (Major Histocompatibility Complex) class II-restricted immune response. On T-lymphocytes, it defines the helper/inducer subset. The mature 55 kd CD4 protein consists of a 372 amino acid extracellular segment composed of four tandem immunoglobulin-like VJ regions. The CD4 molecule is a major receptor for human immunodeficiency virus (HIV), binding directly to the envelope glycoprotein gp120 on HIV, with the co-receptors being CCR5 or CXCR4. It has been shown that the V-like domains are critical for binding with HIV envelope gp120.