The remarkable sensitivity and frequency selectivity of the mammalian cochlea is attributed to a unique amplification process that resides in outer hair cells (OHCs). Although the mammalian-specific somatic motility is considered a substrate of cochlear amplification, it has also been proposed that somatic motility in mammals simply acts as an operating-point adjustment for the ubiquitous stereocilia-based amplifier. To address this issue, we created a mouse model in which a mutation (C1) was introduced into the OHC motor protein prestin, based on previous results in transfected cells. In C1/C1 knockin mice, localization of C1-prestin, as well as the length and number of OHCs, were all normal. In OHCs isolated from C1/C1 mice, nonlinear capacitance and somatic motility were both shifted toward hyperpolarization, so that, compared with WT controls, the amplitude of cycle-bycycle (alternating, or AC) somatic motility remained the same, but the unidirectional (DC) component reversed polarity near the OHC's presumed in vivo resting membrane potential. No physiological defects in cochlear sensitivity or frequency selectivity were detected in C1/C1 or C1/؉ mice. Hence, our results do not support the idea that OHC somatic motility adjusts the operating point of a stereociliabased amplifier. However, they are consistent with the notion that the AC component of OHC somatic motility plays a dominant role in mammalian cochlear amplification.cochlear amplification ͉ mechanosensory ͉ prestin T he remarkable sensitivity, frequency range, and selectivity of the mammalian cochlea have been attributed to a unique amplification process that resides in outer hair cells (OHCs) (1, 2). Two different models have been proposed for cochlear amplification: the mammalian-specific somatic motility (3-5) and the ubiquitous stereociliary motility (6, 7). Somatic motility is known to possess the necessary high-frequency responsiveness (Ͼ79 kHz) (8), whereas the frequency response limitation of stereociliary motility is yet to be established. Somatic motility is believed to be driven by the motor molecule prestin (9). Lack of prestin in mice results in loss of OHC somatic motility, an Ϸ50 dB threshold shift in compound action potentials (CAP), brainstem evoked responses, and otoacoustic emissions, as well as the loss of frequency selectivity (10, 13-15, ʈ, and ** ). These studies provide evidence that prestin is required for OHC somatic motility and cochlear amplification.Recent studies raised the possibility that stereocilia-based amplification may be important in mammalian hearing (16)(17)(18)(19)(20). Because of low-pass filtering of somatic motility by the electrical impedance of the basolateral cell membrane, some believe that the dominant means of amplification must reside in stereociliary motility. In this view, the main function of prestin-based somatic motility is to somehow adjust the operating point of the stereocilia-based amplifier, i.e., that the DC (average or direct current) component of the somatic electromotile response prov...