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Mutations in the CLCNKB gene encoding the human voltage-gated chloride ClC-Kb (hClC-Kb) channel cause classic Bartter's syndrome (BS). In contrast to antenatal BS, classic BS manifests with highly variable phenotypes. The functional severity of the mutant channel has been proposed to explain this phenomenon. Due to difficulties in the expression of hClC-Kb in heterologous expression systems, the functional consequences of mutant channels have not been thoroughly examined, and the genotype–phenotype association has not been established. In this study, we found that hClC-Kb, when expressed in human embryonic kidney (HEK) cells, was unstable due to degradation by proteasome. In-frame fusion of green fluorescent protein (GFP) to the C-terminus of the channel may ameliorate proteasome degradation. Co-expression of barttin increased protein abundance and membrane trafficking of hClC-Kb and markedly increased functional chloride current. We then functionally characterized 18 missense mutations identified in our classic BS cohort and others using HEK cells expressing hClC-Kb-GFP. Most CLCNKB mutations resulted in marked reduction in protein abundance and chloride current, especially those residing at barttin binding sites, dimer interface and selectivity filter. We enrolled classic BS patients carrying homozygous missense mutations with well-described functional consequences and clinical presentations for genotype–phenotype analysis. We found significant correlations of mutant chloride current with the age at diagnosis, plasma chloride concentration and urine calcium excretion rate. In conclusion, hClC-Kb expression in HEK cells is susceptible to proteasome degradation, and fusion of GFP to the C-terminus of hClC-Kb improves protein expression. The functional severity of the CLCNKB mutation is an important determinant of the phenotype in classic BS.