CCL2 and retinal degeneration: A number of association studies have been performed using double knock-out mice lacking both Cx3cr1 and Ccl2 genes [152–155, 163] but their findings have been questioned since many transgenic mice carry the rd8 (retinal degeneration 8) mutation in their Crb-1 (crumbs-like 1) gene that also results in retinal degeneration [164, 166, 167].