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. This evidence concerns the gene EGF and cyst.