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Fig. 1 A large subset of RGCs express GFP in the stable Brn3c:mGFP transgenic line. (A) Schematic drawing of the DNA construct used to generate the Brn3c:mGFP transgenic line. (B) Lateral view of 56 hpf live Brn3c:mGFP transgenic embryo showing GFP expression by RGCs and mechanosensory hair cells (neuromasts) of the lateral line and inner ear. The optic nerve is visible. (C-E) Fixed Brn3c:mGFP transgenic embryos labeled by whole-mount immunohistochemistry. (C) Ventral view of a 42 hpf retina, labeled with anti-GFP. Anterior (nasal) is upwards. GFP expression starts in a small cluster of cells in the ventronasal retina, near the optic fissure. The arrowhead indicates the first GFP-positive axons exiting the eye through the optic stalk. (D) Lateral view of a 44 hpf retina, labeled with anti-GFP in green and zn5 in red. GFP has spread into central retina. The onset of Brn3c:mGFP transgene expression follows that of zn5 by 6 hours. (E) Lateral view of a 72 hpf retina, labeled with anti-GFP. GFP-positive RGCs are distributed uniformly throughout the ganglion cell layer. Scale bars: 200 µm in B; 20 µm in C-E.