CAF22 is the C-terminal 22 kDa domain of agrin. Agrin is cleaved by its specific protease neurotrypsin at two distinct sites, whereas cleavage at the beta site generates a 22 kDa C-terminal fragment (CAF22). In the kidney agrin is part of the basal lamina, where it is the major contributor of the anionic potential of the glomerular basement membrane (GBM). There are indices for a proteolytic activity selectively shedding the C-terminal part of agrin of the GBM. Recently, a 19-fold increase of CAF22 levels in ESDR was observed, which was reduced in patients receiving KT, qualifying CAF22 as effective renal function marker. Additionally, current data support that CAF22 levels are not influenced by inflammatory processes, steroids or by hemodialysis with a standard dialysis membrane.