ATP1A1 and Bartter syndrome: Initially, FXYD2 disease was described as a cause of isolated hypomagnesemia, yet subsequent data on newly discovered patients also show a trend to a Gitelman-like tubulopathy.22, 23 Interestingly, in a recent report of mutations in ATP1A1, encoding the alpha subunit of the Na+-K+-ATPase, hypomagnesemia was the predominant electrolyte abnormality, with hypokalemic alkalosis much less noticable.24