Adult mouse retinas and brain sections were stained, processed, and imaged as previously described (Tudor C Badea, Cahill, et al., 2009 (link); Tudor Constantin Badea & Nathans, 2004 (link); T. C. Badea, Wang, & Nathans, 2003 (link)). Mice were anesthetized with ketamine and xylazine and fixed by intracardiac perfusion with 4% Paraformaldehyde (PFA). Retinas and brains were dissected and post-fixed in 4% PFA for 45 minutes or 2 hours respectively. Retinas were flat-mounted and brains were sliced coronally at 200 μm thickness using a vibratome (Leica). Retinal flat-mounts and brain sections were washed in PBS and heat inactivated in a water bath at 65°C for one hour to inactivate the endogenous AP activity. Staining was performed in AP buffer (0.1mM Tris, 0.1M NaCl, 50mM MgCl2, pH9.5), with 0.34 mg/ml nitroblue tetrazolium (NBT) and 0.35 mg/ml 5-bromo-4-chloro-3indolyl-phosphate (BCIP; Roche), for 1–12 hours at room temperature with gentle agitation. Following multiple washes in PBS – 0.1% Tween 20 and an ethanol dehydration series, stained retinas and brains were mounted in benzyl:benzoate: benzyl alcohol (BB:BA, 2:1). Low resolution images of brain sections were acquired on a Zeiss Discovery V8 Stereomicroscope, using a Zeiss Axiocam MRC color camera, and Axiovision software. Color images of retinal flat-mounts were acquired using a Zeiss Imager.Z2.