Whereas cone photoreceptor cells can survive for many years following the loss of rod photoreceptor cells in human presentations of inherited retinal disease (Milam et al., 1998; Banin et al., 1999) and are therefore a potential target of optogenetic therapy, testing such an approach in rd1 mice may be of limited use as they maintain only a single row of cone cells at 3 weeks of age. This evidence concerns the gene PDE6B and Abnormal retinal morphology.