ESPN encodes espin, a multifunctional actin-bundling protein essential for inner ear mechanotransduction. ESPN regulates the organization, dynamics, and signaling of actin filament-rich microvilli in sensory cells 1. The protein is required for assembly and stabilization of stereociliary parallel actin bundles, playing a crucial role in formation and maintenance of hair cell stereocilia 1. In extrastriolar hair cells, ESPN is necessary for targeting MYO3B to stereocilia tips and regulating stereocilia diameter and staircase formation. ESPN promotes actin elongation within stereocilia, directly supporting mechanotransduction function 1. Pathogenic ESPN variants cause autosomal recessive deafness (DFNB36) with or without vestibular involvement, and have been associated with Usher syndrome type 1M, a syndromic form combining progressive hearing loss with retinitis pigmentosa. The protein's role in actin bundle organization makes it critical for the structural and functional integrity of sensory organelles. Mutations disrupting ESPN function impair stereocilia morphogenesis and mechanotransduction, leading to sensorineural hearing loss across the spectrum from early-onset profound deafness to progressive hearing impairment.