GDNF and renal agenesis: This question is difficult to address, but in GDNF (glial cell-derived neurotrophic factor) heterozygous mice, a model with low nephron number and in which 20% of animals have unilateral renal agenesis, single kidney nephron numbers were found to be identical in mice born with one or two kidneys.68 Although glomerular filtration rates (GFR) were similar, salt and water handling were different, suggesting possible alterations in nephron function in the mice with unilateral renal agenesis.