Aqp2F204V/F204V mice have dramatically increased urine production, in some cases producing an amount of urine in 24 h that exceeds their body weight, compared to their heterozygous or wild-type littermates. Such loss of water would rapidly lead to dehydration were it not compensated by increased water intake. Indeed, mutant mice also dramatically increase their water intake (Figure 1C) compared to their heterozygous or wild-type littermates. This phenotype—increased urinary output and water intake—showed complete concordance with homozygosity of the F204V mutation in the 58 animals tested.