Stereocilia of the inner ear play an integral role in the mechanotransduction of sound. Their structural support is derived from actin filaments and actin-binding proteins. We have identified a novel actin-binding protein, 2E4-kaptin (KPTN), which appears to be involved in this structural network. Using double label immunofluorescence, we now show that KPTN extends beyond the barbed ends of actin filaments at the tips of stereocilia, and using cloned human cDNA, we mapped KPTN to chromosome 19q13.4. A combination of FISH, radiation hybrid mapping and YAC screening localized KPTN between markers D19S412 and NIB1805, making this gene an excellent functional and positional candidate for DFNA4, a form of autosomal dominant non-syndromic hearing loss. We identified a second family with inherited deafness that also maps to the DFNA4 region. To screen KPTN for deafness-causing mutations, we first determined its genomic structure and then completed a mutational analysis by direct sequencing and SSCP in affected family members. Although no deafness-causing mutations were identified in the coding region, KPTN remains an excellent candidate gene for hearing loss ; by synteny, its murine orthologue also remains a candidate gene for the Nijmegan waltzer (nv) mouse mutant, which has vestibular defects and a variable sensorineural hearing loss. Mechanotransduction of hearing is mediated through hair cells of the sensory epithelium of the inner ear (Hudspeth, 1997). These cells lie below the tectorial membrane, which oscillates in response to sound waves, displacing stereocilia and opening gated ion channels to give rise to an electrical signal. Each hair cell has a unique complement of stereocilia, varying in length and