Inner ear supporting cells from newborn mice harbor a latent capacity for some regenerative responses, but these disappear within the first few weeks of life. This observation provides an experimental window this proposal will exploit 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 perinatal organ of Corti; and investigating the changes in the chromatin, the epigenome, that lead to the failure of regeneration in the adult inner ear.