ROBO2 and hydronephrosis: To determine if an enlarged ureterovesical junction was the primary congenital defect at an early developmental stage in Robo2 mutant embryos and later caused reflux and hydronephrosis, we examined the structure of the ureterovesical junction and entire upper urinary tract by micro-ultrasonography in 30 Robo2 newborn mice with antenatal hydronephrosis (Video S7 and Video S8).