Probable serine protease that plays a role in hearing. Acts as a permissive factor for cochlear hair cell survival and activation at the onset of hearing and is required for saccular hair cell survival. Activates ENaC (in vitro). Strongly expressed in liver, cochlea, brain, cerebellum, spleen, lung, and muscle and at a lower degree in retina, kidney, and heart. Expressed in the spiral ganglion, the cells supporting the organ of Corti and the stria vascularis. Isoform 2 is strongly expressed only in the cochlea with very faint expression in the cerebellum, spleen and muscle. Undergoes autoproteolytic activation. Belongs to the peptidase S1 family.