Recently a number of eukaryotic cell surface antigens have been found to be evolutionary related [1,2,3]. The proteins known to belong to this family are listed below:

• Mammalian antigen CD9 (MIC3); a protein involved in platelet activation and aggregation.
• Mammalian leukocyte antigen CD37, expressed on B lymphocytes.
• Mammalian leukocyte antigen CD53 (OX-44), which may be involved in growth regulation in hematopoietic cells.
• Mammalian lysosomal membrane protein CD63 (Melanoma-associated antigen ME491; antigen AD1).
• Mammalian antigen CD81 (Cell surface protein TAPA-1), which may play an important role in the regulation of lymphoma cell growth.
• Mammalian antigen CD82 (Protein R2; Antigen C33; Kangai 1 (KAI1)), which associates with CD4 or CD8 and delivers costimulatory signals for the TCR/CD3 pathway.
• Mammalian antigen CD151 (SFA-1; Platelet-endothelial tetraspan antigen 3 (PETA-3)).
• Mammalian TM4SF2 (Cell surface glycoprotein A15; TALLA-1; MXS1).
• Mammalian TM4SF3 (Tumor-associated antigen CO-029).
• Mammalian TM4SF6 (Tspan-6; TM4-D).
• Mammalian TM4SF7 (Novel antigen 2 (NAG-2); Tspan-4).
• Mammalian Tspan-2.
• Mammalian Tspan-3 (TM4-A).
• Mammalian Tetraspan NET-5.
• Schistosoma mansoni and japonicum 23 Kd surface antigen (SM23 / SJ23).

These proteins share the following characteristics: they all seem to be type III membrane proteins (type III proteins are integral membrane proteins that contain a N-terminal membrane-anchoring domain which is not cleaved during biosynthesis and which functions both as a translocation signal and as a membrane anchor); they also contain three additional transmembrane regions, at least seven conserved cysteines residues, and are of approximately the same size (218 to 284 residues). These proteins are collectively know as the 'transmembrane 4 superfamily' (TM4) because they span the plasma membrane four times. A schematic diagram of the domain structure of these proteins is shown below.

 +-+-----+-------+----+-----+-----+-----------------------------+-----+----+
 | | TMa | Extra | TM2| Cyt | TM3 | Extracellular               | TM4 | Cyt|
 +-+-----+-------+----C-----C-----+--------CC---------C-----C---+-----C----+
                     *********

Cyt       : cytoplasmic domain.
TMa       : transmembrane anchor.
TM2 to TM4: transmembrane regions 2 to 4.
'C'       : conserved cysteine.
'*'       : position of the pattern.

As a signature for these proteins, we selected a conserved region that includes two cysteines and seems to be located in a short cytoplasmic loop between two transmembrane domains.