Comparing proteins eluted after protein-A immunoadsorption, we identified a serum form of CASK, which was detected only in the sera of patients with rFSGS, but not in healthy individuals, in kidney-transplant patients without proteinuria, patients with significant proteinuria caused by diabetic nephropathy, minimal change disease, or membranous glomerulonephritis. Here, CASK is linked to membranous glomerulonephritis.