POU3F4 and X-linked mixed deafness with perilymphatic gusher: Among the four types of X-linked HL (DFN2, DFN3, DFN4, and DFN6), X-linked deafness type 2 (DFNX2, also known as DFN3, OMIM #304400) accounts for about 50% of X-linked deafness cases and is associated with pathogenic sequence alterations in the POU3F4 gene (BRAIN-4, BRN4, OMIM *300039) (Phippard et al., 1999), which codes for a highly conserved protein belonging to the POU family of transcription factors.