Glycosaminoglycan (GAG) is a macromolecule found on the surface of eukaryotic cells which is thought to play a role in the cell's recognition of other cells or of a substrate. It consists of a network of long, branched chains made up of repeating units of disaccharides which contain amino groups.

It is also a protein component of the capsid of a retrovirus and can act as group-specific antigens to the virus' host, thus causing immune reactions.

The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.