The majority of putative alpha retinal ganglion cells (RGCs) are Isl2-GFP+. (A-C) Immunofluorescent staining of the putative alpha cell marker, SMI-32 (B), and GFP (A) in a flat mount Isl2-GFP retina reveals a high degree of co-localization in both cell bodies (arrows) and axons (arrowheads). bar, 100 μm. (D-F) Immunofluorescent staining of the intrinsically photosensitive RGC marker, melanopsin (E), and GFP (D) in an Isl2-GFP retina reveals a subset of co-labeled cells (arrows) and single-labeled cells (double arrowheads). bar, 50 μm.