Although hair cell regeneration does not occur in mammals, newborn mice harbor a latent capacity for some regenerative responses. However, this capability disappears within the first few weeks of life. This observation provides an experimental window that this proposal exploits to address fundamental questions about the failure of hair cell regeneration in mammals. Specifically, we propose experiments to identify those changes in the genetic material, the chromatin, that are responsible for orchestrating the differentiation of new hair cells within the newborn organ of Corti in the inner ear, and to investigate the changes in the chromatin that lead to the failure of regeneration in the adult mammalian inner ear.