In different models (Bbs2, Bbs4 and Bbs12) retinal degeneration has been shown to be progressive, first evident as a thinning of the outer nuclear layer (photoreceptors) by as early as 6 weeks of age and characterized by complete loss of the outer segment in older animals (7 months old in Bbs4 KO animals; [54]). This evidence concerns the gene BBS2 and retinal degeneration.