RHO and retinal degeneration: To confirm the toxicity was specific to rhodopsin, and rule out the possibility that metformin could accelerate photoreceptor cell death in the presence of another cause of retinal degeneration, we investigated the effect of metformin in the rhodopsin knock out (KO) (Rho−/−) mouse model that does not elaborate ROS and its photoreceptors die within three months (44).