AQP7-KO mice display a clear phenotype of adult-onset obesity and hyperinsulinemia (Maeda et al., 2004; Hibuse et al., 2005; Matsumura et al., 2007), but the impact of AQP7 loss-of-function homozygous mutations in human obesity and diabetes remains controversial. The gene discussed is AQP7; the disease is obesity due to melanocortin 4 receptor deficiency.