BACKGROUND: Pathogenic variants of USH1C, encoding a PDZ-domain-containing protein called harmonin, have been known to cause autosomal recessive syndromic or nonsyndromic hearing loss (NSHL). We identified a causative gene in a large Korean family with NSHL showing a typical pattern of autosomal dominant (AD) inheritance. METHODS: Exome sequencing was performed for five affected and three unaffected individuals in this family. Following identification of a candidate gene variant, segregation analysis and functional studies, including circular dichroism and biolayer interferometry experiments, were performed. RESULTS: A novel USH1C heterozygous missense variant (c.667G>T;p.Gly223Cys) was shown to segregate with the NSHL phenotype in this family. This variant affects an amino acid residue located in the highly conserved carboxylate-binding loop of the harmonin PDZ2 domain and is predicted to disturb the interaction with cadherin-related 23 (cdh23). The affinity of the variant PDZ2 domain for a biotinylated synthetic peptide containing the PDZ-binding motif of cdh23 was approximately 16-fold lower than that of the wild-type PDZ2 domain and that this inaccessibility of the binding site was caused by a conformational change in the variant PDZ2 domain. CONCLUSIONS A heterozygous variant of USH1C that interferes with the interaction between cdh23 and harmonin causes novel AD-NSHL.