The FXYD domain-containing ion transport regulator 12 (FXYD12) mRNA is mainly distributed in kidneys and intestines of fish. In co-immunoprecipitation experiments, FXYD12 was shown to associate with the Na(+)/(K+)-ATPase (NKA) alpha-subunit in the intestines of two closely related medakas, Oryzias dancena and O. latipes. These results suggests that FXYD12 may play a role in modulating NKA activity in the intestines following salinity changes in the maintenance of internal homeostasis.