GDNF and renal agenesis: For instance, hypertension has been proposed to be at least partially explained by insufficient nephron number (Brenner et al., 1988), whereas renal agenesis is a relatively frequent congenital defect in humans, mirroring the GDNF knockout in mice, that results in the formation of tiny rudimentary ductal trees in the kidney (Pichel et al., 1996).