Whole mount immunostaining for BRN3A was performed as we recently described [18 (link)]. Briefly, fixed eyecups were incubated in PBS buffer containing 0.5% Triton-X100 and 2% donkey serum (Jackson ImmunoResearch Lab, MS) for 1.5 h at room temperature. They were then transferred into the same buffer containing mouse anti-BRN3A (Millipore, Cat.#MAB1585, 1:50) and incubated overnight at 4 °C. The eyecups were thoroughly rinsed in PBS buffer with 0.5% Triton-X100; they were then fixed for an additional 10 min in 4% paraformaldehyde and rinsed again. The eyecups were whole-mounted onto Fisher Plus slides and incubated in 2% Triton-X100 and 2% donkey serum with a secondary antibody (Alexa-594-conjugated donkey-anti-mouse, 2 μg/ml, Jackson ImmunoResearch Lab, MS) for 2 h at room temperature. The whole mounts were rinsed in PBS buffer and stained with 300 ng/ml DAPI for 5 min at room temperature. After a final wash with PBS buffer, the slides were coverslipped with Immu-Mount (ThermoFisher) and used for fluorescence microscopy. Images were acquired with a Nikon fluorescence microscope using a 20× objective lens and analyzed by Nikon Elements software.