A nalysis of Mendelian Mg²⁺ wasting disorders helps us to unravel the mechanisms of Mg²⁺ homeostasis. In this issue of the JCI, Glaudemans and colleagues show that mutations in voltage-gated K⁺ channel subtype 1.1 (Kv1.1) cause autosomal dominant hypomagnesemia in humans (see the related article beginning on page 936). Interestingly, other mutations in the same protein cause the neurological disease episodic ataxia type 1. The authors show, using cells with heterologous expression of the wild-type and mutant channels, that the mutant channel is dysfunctional and speculate that Mg²⁺ wasting results from changes in apical membrane voltage along the nephron. Mechanisms by which the apical voltage is generated and how Kv1.1 fits within this context are discussed herein.