Syndecans are single transmembrane domain proteins that are thought to act as coreceptors, especially for G protein-coupled receptors.

[3] The most obvious differences between syndecans include (together with differences in distribution) the subclassification of the family depending on the existence of GAG binding sites either at both ends of the ectodomain (syndecan-1 and - 3) or at the distal part only (syndecan-2 and -4) and a relatively long Thr-Ser-Pro-rich area in the middle of syndecan- 3's ectodomain.

The difference in size of the syndecans is credited to the variable length of exon 3, which encodes a spacer domain [1, 14].

The linker is composed of four saccharides, first one being xylose, which is an unusual sugar in a unique place, attached to serine of the protein core and sequentially followed by two galactose and a β-D-glucuronic acid [1, 12].

In tissues, it is specific to the brain and expressed at low levels in liver, kidney, lung and small intestine.

Functionality of syndecan is contributed by glycosaminoglycans which help in the interaction with different extracellular ligands.

Depending upon the cellular localization of syndecan, glycosaminoglycans have different structures to accommodate the functional needs of the region.

Glycosaminoglycans attached to the syndecan help binding of the various growth factors for activation of important cellular signaling mechanisms.

For example, at the site of tissue injury, the soluble syndecan-1 ectodomains are cleaved by heparanases, producing heparin-like fragments that activate bFGF [13].

It associates with intracellular actin cytoskeleton and helps maintain normal epithelium sheet morphology The syndecan proteins can contain the following protein domains, Syndecan-4 is upregulated in endometriosis and inhibition of syndecan-4 in human endometriotic cells results in a reduction of invasive growth in vitro and changes in matrix metalloproteinase expression.

Similarly, syndecan 1 expression has been linked with low differentiation in squamous cell carcinoma of the head and neck [15].

For example, syndecan 1 expression is increased in ductal breast carcinomas and is associated with factors of angiogenesis and lymphangiogenesis [5].