BSND encodes barttin, a regulatory subunit that forms heteromeric chloride channels with CLCNKA and CLCNKB in the kidney and inner ear 1. The protein functions as a chaperone that facilitates proper trafficking of these channel complexes to the basolateral plasma membrane and enables their transition to an ion-conducting state 1. In the kidney, BSND is expressed in the thin and thick ascending limbs of the loop of Henle, where it is essential for chloride reabsorption and maintaining systemic electrolyte balance 12. In the inner ear, BSND is expressed in dark cells and is crucial for establishing the endocochlear potential necessary for normal hearing 1. Loss-of-function mutations in BSND cause Bartter syndrome type IV, characterized by severe renal salt wasting, sensorineural deafness, and often kidney failure 12. Some mutations can also cause isolated nonsyndromic hearing loss without the full Bartter syndrome phenotype 3. Functional variants like V43I show partial loss-of-function effects and may contribute to blood pressure regulation differences among populations 4. BSND is also expressed in salivary gland striated ducts and serves as a diagnostic marker for oncocytic tumors in both renal and salivary tissues 56.