Corticosteroid hormone-induced factor (CHIF) is a short epithelial-specific protein that is independently induced by aldosterone and a high-K+diet. It is a member of the FXYD family of single-span transmembrane proteins that include phospholemman, Mat-8, and the γ-subunit of Na+-K+-ATPase. A number of studies have suggested that these proteins are involved in the regulation of ion transport and, in particular, functionally interact with the Na+-K+-ATPase. The present study describes the characterization, targeted disruption, and phenotypic analysis of the mouse CHIF gene. The CHIF knockout mice are viable and not distinguishable from wild-type littermates under normal conditions. Under K+loading, they have a twofold higher urine volume and an increased glomerular filtration rate. Similar but smaller effects are observed in mice fed a low-Na+diet. Treating K+-loaded mice for 10 days with furosemide resulted in lethality in the knockout mice (17 of 39) but not in the wild-type group (1 of 39). The data are consistent with an effect of CHIF on the Na+-K+-ATPase that is specific to the outer and inner medullary duct, its major expression site.