Figure 1. Schematic illustration of a two-dimensional model of human CD81 membrane receptor.

The extracellular domain contains two loops, the small extracellular loop (SEL) and large extracellular loop (LEL). The CD81 receptor contains the rhodopsin C-terminal 9-residue peptide sequence (shown underlined) at its C-terminus as the tag to facilitate detection and purification of the membrane protein using the monoclonal antibody rho-1D4.