Down-regulation of the BMP/Smad1 pathway occurs during the age-related decline in axon growth potential and, in adults, blockade of BMP signalling, by either pharmacological inhibition or knockdown of Smad1, arrests the initiation and elongation of DRGN neurites [13], whereas activation of Smad1 by intrathecal injection of AAV-BMP4 stimulates DRGN axon regeneration through mouse DC lesions [13]. The gene discussed is BMP4; the disease is dyskeratosis congenita.