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Background Heterozygous variants in SLC12A6 have recently been shown to cause dominant Charcot-Marie-Tooth disease (CMT). We aim to characterise the phenotype of patients with previously reported and novel heterozygous variants in the gene and understand any genotype-phenotype correlation.Methods Patients were clinically and genetically assessed in sites from Europe, Australia, Brazil and the USA. All patients underwent whole exome or whole genome sequencing. Variants were classified using American College of Medical Genetics and Genomics criteria.Results Twenty-three individuals from 13 families carried nine variants classified either as pathogenic/likely pathogenic or variants of uncertain significance segregating in multiple family members, including five novel variants. Forty-eight percent (11/23) were male with a mean age of disease onset of 15.7 years (range 1–45 years). Clinical phenotype varied dramatically with genotype; Arg207His and Ser647Pro caused a severe childhood-onset, sensory and motor, conduction-slowing neuropathy, whereas Gly552Asp caused a mild, adult-onset, sensory-predominant neuropathy, Thr991Ala an infantile-onset motor neuropathy, and the Met282Lys/Gly286Cys locus a complex, axonal neuropathy.Conclusions Heterozygous variants in SLC12A6 can cause CMT of all clinical phenotypes, severity and age of onset, depending on the genotype. Such phenotypic diversity has not been described for any other CMT gene, and more work is needed to understand disease mechanisms to guide future therapeutic options.