Abstract-We identified a new kindred with the familial syndrome of hypertension and hyperkalemia (pseudohypoaldosteronism type II or Gordon's syndrome) containing an affected father and son. Mutation analysis confirmed a single heterozygous G to C substitution within exon 7 (1690GϾC) that causes a missense mutation within the acidic motif of WNK4 (564DϾH). We confirmed the function of this novel mutation by coexpressing it in Xenopus oocytes with either the NaCl cotransporter (NCCT) or the inwardly rectifying K-channel (ROMK). Wild-type WNK4 inhibits 22 Na ϩ flux in Xenopus oocytes expressing NCCT by Ϸ90% (PϽ0.001), whereas the 564DϾH mutant had no significantly inhibitory effect on flux through NCCT. In oocytes expressing ROMK, wild-type WNK4 produced Ͼ50% inhibition of steady-state current through ROMK at a ϩ20-mV holding potential (PϽ0.001). The 564DϾH mutant produced further inhibition with steady-state currents to some 60% to 70% of those seen with the wild-type WNK4. Using fluorescent-tagged NCCT (enhanced cyan fluorescent protein-NCCT) and ROMK (enhanced green fluorescent protein-ROMK) to quantify the expression of the proteins in the oocyte membrane, it appears that the functional effects of the 564DϾH mutation can be explained by alteration in the surface expression of NCCT and ROMK. Compared with wild-type WNK4, WNK4 564DϾH causes increased cell surface expression of NCCT but reduced expression of ROMK. This work confirms that the novel missense mutation in WNK4, 564DϾH, is functionally active and highlights further how switching charge on a single residue in the acid motif of WNK4 affects its interaction with the thiazide-sensitive target NCCT and the potassium channel ROMK.