SPP1 and autosomal dominant polycystic kidney disease: The levels of osteopontin (OPN), which is expressed by renal epithelial tubular cells and excreted into urine, were reduced in biallelic ADPKD gene (PKD2−/−) porcine knockout cells and urinary OPN-excretion levels were lower in patients with rapidly progressive disease than in slow progressors [38] (Table 2).