The retinoblastoma protein is involved in the growth and development of mammalian hair cells of the cochlea, and appears to be related to the cells' inability to regenerate. Embryonic hair cells require pRb, among other important proteins, to exit the cell-cycle and stop dividing, which allows maturation of the auditory system. Once wild-type mammals have reached adulthood, their cochlear hair cells become incapable of proliferation. In studies where the gene for pRb is deleted in mice cochlea, hair cells continue to proliferate in early adulthood. Though this may seem to be a positive development, pRb-knockdown mice tend to develop severe hearing loss due to degeneration of the organ of Corti. For this reason, pRb seems to be instrumental for completing the development of mammalian hair cells and keeping them alive. However, it is clear that without pRb, hair cells have the ability to proliferate, which is why pRb is known as a tumor suppressor. Temporarily and precisely turning off pRb in adult mammals with damaged hair cells may lead to propagation and therefore successful regeneration.  Suppressing function of the retinoblastoma protein in the adult rat cochlea has been found to cause proliferation of supporting cells and hair cells. pRb can be downregulated by activating the sonic hedgehog pathway, which phosphorylates the proteins and reduces gene transcription.