PDE6B and retinal degeneration: Wild-type C3H strains exhibit retinal degeneration, due to a recessive mutation in the Pde6b gene (Pde6brd1), and hence we employed the line, C3H.Pde6b+ (ref. 11), congenic for the BALB/c region encompassing Pde6b and which does not exhibit retinal degeneration enabling screening for visual abnormalities.