PDE6B and retinitis pigmentosa 1: Additionally, one study identified two candidate genes (NRL and PDE6B) for targeted gene therapy RP research, with the important caveat that DNA methylation must be considered for reprogramming the transition of adult rod photoreceptor cells to cone cells; adult murine models subjected to Nrl/Pde6b inactivation showed only partial reprogramming which was insufficient to alter DNA methylation patterns, and incomplete RP symptom alleviation.49