Most X-linked nonsyndromic hearing loss is caused by various types of mutations of the POU domain class 3 transcription factor 4 gene (POU3F4). We found five unique missense and frameshift truncation and extension mutations in Korean patients. Two missense mutations (p.Thr211Met and p.Gln229Arg) disturbed transcriptional activity. Two frameshift extension mutations (p.Thr354GlnfsX115 and p.X362ArgextX113) were located outside of POU domain and nuclear localization signal (NLS) at the C-terminus. POU3F4 protein levels were low and could be restored by MG132, a proteasome inhibitor, in vitro. These mutant POU3F4 proteins were exclusively localized to the cytoplasm and did not have transcriptional activity. Frameshift mutation (p.Leu317PhefsX12) in POU3F4 leads to the truncation of the C-terminal 44 amino acids spanning the POU domain and NLS. This frameshift truncation mutant protein was located in both the nucleus and cytoplasm and was present at low protein levels. This mutant was also transcriptionally inactive, even in the presence of MG132. From these results, we conclude that frameshift truncation and extension mutations in the C-terminus of POU3F4 lead to cytoplasmic localization and subsequent proteosomal degradation due to structural aberrations, which cause transcriptional inactivity and thus nonsyndromic hearing loss.