SLC12A3 and Hypokalemia: Therefore, his phenotype is consistent with his type of mutation in CLCNKB. II-2 and II-4 carry dual homozygous mutations in SLC12A3 and CLCNKB. II-4 has renal dysfunction and very severe hypokalemia (serum K+ 1.8 mmol/L), which both indicate his phenotype is severe.