Retinal ganglion cells (RGCs) are marked in Isl2-GFP mice. (A-C) Immunofluorescent staining of the RGC marker Brn3a reveals co-localization (arrows) in a subset of Isl2-GFP+ cells in a P15 retina. bar, 50 μm. (D) Schematic of retrograde tracing strategy to specifically label RGCs. (E and F) Fluorescent micrographs of retrogradely labeled RGCs (red) in Isl2-GFP mice in cross-section (E) and flat mount (F) reveals subsets of co-labeled cells (arrows), retrogradely-labeled-only cells (arrowheads) and GFP-only cells (double arrowheads). bar, 50 μm.