An experimental model revealed that Pax-2−/− mutant mice lack kidneys and a ureter because the ureteric bud fails to branch from the mesonephric duct, while haplo-insufficiency (a lack of functional protein due to a mutated allele) in Pax-2+/− mutant mice is associated with renal hypoplasia and sometimes hydro-ureteronephrosis consistent with the presence of vesico-ureteric reflux [13]. Here, PAX2 is linked to renal hypoplasia.