FXYD domain-containing ion transport regulator 11 (FXYD11) is a putative regulatory subunit of the Na(+)/K(+)-ATPase (NKA) pump. FXYD11 is expressed predominantly in the gills of euryhaline teleosts, such as the spotted scat, Scatophagus argus. It regulates NKA activity through protein-protein interactions. The regulation of NKA and FXYD11 is of critical importance for osmotic homeostasis. The expression and activity of NKA, as well as FXYD11 mRNA expression in gills have been shown to respond to different environmental salinity by dual-labeling immunohistochemistry and quantitative PCR (RT-qPCR) methods, indicating that there is an interaction between NKA and FXYD.