Each chain consists of six domains; a large N-terminal ten-bladed β-propeller followed by two 10CC domains (10CC-a and 10CC-b), two polycystic kidney disease (PKD) domains (PKD1 and PKD2) with an FN-III topology, and an unpredicted C-terminal membrane proximal domain (residues 958-1066) that we termed SorCS membrane proximal (SoMP) (Fig. 1c). The gene discussed is PKD1; the disease is polycystic kidney disease.