*8.5. RD3*

Mutations in *RD3* result in LCA12 in humans, a severe retinal degeneration in mice and rod–cone dysplasia type 2 (*rcd2*) in Collie dogs [178–180]. RD3 is thought to act as competitor for guanylyl cyclase-activating proteins, preventing premature cyclase activity in inner segments [181].

Collie dogs have a 22 bp insertion in the *Rd3* gene resulting in altered amino acids and an extended open reading frame [182]. Three splice variants of the gene were identified in the canine retina. In the *Rd3*-mutant Collie, the rod and cone photoreceptors outer segments do not fully develop and the retina degenerates rapidly. By 6 months, there are no photoreceptor inner or outer segments and the outer nuclear layer contains only one layer of nuclei [183].
