We report the cloning of the Ocp2 gene encoding OCP-II from a guinea pig organ-of-Corti cDNA library. The predicted open reading frame encodes a protein of 163 amino acids with an estimated molecular mass of 18.6 kDa. A homology search revealed that Ocp2 shares significant sequence similarity with p15, a subunit of transcription factor SIII that regulates the activity of the RNA polymerase II elongation complex. The Ocp2 messenger RNA is expressed abundantly in the cochlea while not significantly in any other tissues examined, including brain, eye, heart, intestine, kidney, liver, lung, thigh muscle, and testis, demonstrating that the expression of this gene may be restricted to auditory organs. A polyclonal antiserum was raised against the N-terminal region of OCP-II. Immunohistochemical staining of paraffin-embedded sections of the cochlea showed that OCP-II is localized abundantly in nonsensory cells in the organ of Corti; in addition, it was also detected, at a lower concentration, in vestibular sensory organs, as well as auditory and vestibular brain stem nuclei. The Ocp2 gene was mapped to mouse chromosome 4 as well as 11. Our results suggest that OCP-II may be involved in transcription regulation for the development or maintenance of specialized functions of the inner ear.