Syndecans [1,2] (from the greek syndein; to bind together) are a family of transmembrane heparan sulfate proteoglycans which are implicated in the binding of extracellular matrix components and growth factors. Syndecans bind a variety of molecules via their heparan sulfate chains and can act as receptors or as co-receptors.

Structurally, these proteins consist of four separate domains:

 a) A signal sequence;
 b) An extracellular domain (ectodomain) of variable length and whose sequence
    is not  evolutionary  conserved  in  the  various  forms of syndecans. The
    ectodomain contains  the  sites  of  attachment  of  the  heparan  sulfate
    glycosaminoglycan side chains;
 c) A transmembrane region;
 d) A  highly  conserved  cytoplasmic  domain of about 30 to 35 residues which
    could interact with cytoskeletal proteins.

The proteins known to belong to this family are:

• Syndecan 1.
• Syndecan 2 or fibroglycan.
• Syndecan 3 or neuroglycan or N-syndecan.
• Syndecan 4 or amphiglycan or ryudocan.
• Drosophila syndecan.
• Caenorhabditis elegans probable syndecan (F57C7.3).

The signature pattern that we developed for syndecans starts with the last residue of the transmembrane region and includes the first 10 residues of the cytoplasmic domain. This region, which contains four basic residues, could act as a stop transfer site.