GUCY2D and Blindness: Based on previous animal studies (36, 37) and the clinical cases describing loss of function in RetGC1 associated with a recessive blindness, GUCY2D Leber’s congenital amaurosis (38, 39, 40), RetGC1 is the main isozyme of RetGC providing most of cGMP production in mammalian photoreceptors, including human rods and cones.