In a Dutch pedigree suffering from autosomal dominant nonsyndromic hearing impairment (ADNSHI), linkage was found to the locus for DFNA15, with a two-point logarithm of the odds (LOD) score of 5.1. Sequence analysis of the POU4F3 gene that is involved in DFNA15 revealed the presence of a missense mutation (c.865C>T), segregating with the deafness in this family. The mutation is predicted to result in the substitution of a phenylalanine residue for a leucine residue (p.L289F) in the POU homeodomain of the transcription factor POU4F3. Mutation analysis of the POU4F3 gene in 30 patients suffering from dominantly inherited hearing impairment revealed a second novel missense mutation (c.668T>C), resulting in the substitution of a proline for a leucine residue (p.L223P) within the POU-specific DNA-binding domain of the protein. In a computer model describing the structure of the two DNA-binding domains, the alterations are predicted to affect the tertiary structure of these domains. Transient transfection studies showed that whereas the wild-type POU4F3 is located almost exclusively in the nucleus, part of the mutant proteins was also present in the cytoplasm. In addition, both mutant proteins showed greatly reduced capability for binding to DNA as well as transcriptionally activating reporter gene expression. Together, our results describe the identification of the first missense mutations in POU4F3 causing DFNA15. Furthermore, mutations in this gene do not seem to be a rare cause of hearing impairment in the Dutch population, and the POU4F3 gene may thus be suitable for implementation in diagnostic testing. Hum Mutat 29(4), 545–554, 2008. © 2008, Wiley-Liss, Inc.