ERBB2 and autosomal dominant polycystic kidney disease: EGF and EGF-like ligands are secreted into the apical medium of cultured cystic epithelial cells and are present in cyst fluid from ADPKD patients.[12], [13] Thus, in cystic epithelia, both receptors (ErbB1 and ErbB2) and ligands are expressed on the same side of the cell leading to sustained mitogenic signaling.