The principal molecular structure of basement membranes has been elucidated during the past two decades and it was shown that for example in the glomerular basement membrane (GBM) HS-PG are responsible for the selective filtration process. Removal of HS-PG will lead to proteinuria. Whereas previously perlecan was the only known basement membrane HS-PG, there is now evidence that (at least two) other basement membrane HS-PG do exist: Agrin, originally discovered as an important component of the neuromuscular junction and a novel small HS-PG, that was isolated from human aorta and kidney. This HS-PG, with a molecular weight of 80-200kD (aorta) and 30-160kD (kidney) and a core protein size of 24kD or 22kD, respectively, was localized by immunohisto-hemistry to the basement membrane. Amino acid sequence analysis of tryptic peptides indicate, that this small HS-PG is clearly distinct from perlecan and agrin.