MAP2K2 and Hydroureter: Hoxb7CreGFP;Mek1F/F;Mek2-/- (henceforth called “dko” for double knock-out) mice were born in the expected Mendelian ratio (data not shown) but died within 72 h due to obvious renal defects, including severe renal hypodysplasia and sporadic hydroureters (16%) (Figure 3C, E and G).