PDE6A and retinitis pigmentosa 1: Our previous detailed characterization of the PDE6A-/- dog phenotype showed that in the central retina the number of rod nuclei per 100μm of retinal length was decreased from about 240 nuclei in control retina to 125 nuclei in the affected dogs [10] A potential limitation of the Kjellström et al study was that the RP patients were reported to be of Iraqi origin while the controls were healthy (presumably local) volunteers.