A protein with four transmembrane domains, small intracellular amino- and carboxyl-terminal regions, and two extracellular loops, a small one (EC1) and a larger one (EC2) of about 100 residues. The EC2 loop contains at least 4 cysteine residues, including a highly conserved 'CCG' motif. [ http://en.wikipedia.org/wiki/Tetraspanin ]