Cone degeneration in 18-month-old Rp2-KO mice. Age-matched WT C57/Bl6 mice were used as controls. (A) M- and S-opsin staining of retinal sections. Markedly reduced numbers of M- and S-opsin expressing cells in the KO retina were observed compared with those in the WT retina. (B) Rhodopsin staining of retinal sections. No obvious differences were observed between KO and WT mice in rhodopsin expression and its intracellular localization, and thickness of rod-dominant photoreceptor layer. Scale bar 20 μm. OS, outer segments; IS, inner segments; ONL, outer nuclear layer; INL, inner nuclear layer.