The enucleated eyeballs were immersed in fixative for 1 hour, bisected, and postfixed for 3 hours. The eye cups were incubated with 30% sucrose at 4°C overnight and embedded in optical cutting temperature compound (O.C.T.; Sakura Finetec, Torrance, CA). Sections (10 μm thick) were obtained along the vertical meridian through the optic nerve head. Standard immunohistochemical procedures were performed with primary rabbit antibodies against Rbpms and mouse III β-tubulin (TUJI, 1:200, mouse; Covance, Emeryville, CA), and FITC-conjugated anti-rabbit and rhodamine-conjugated anti-mouse secondary antibody. Negative control sections were incubated without primary or secondary antibodies. For quantification, at least two retinal sections from each eye were collected. For each retinal section, the numbers of cells in the RGC layer labeled with immunoreactivity of (1) Rbpms and III β-tubulin, (2) Rbpms but no III β-tubulin, and (3) III β-tubulin but no Rbpms were counted. Retinal sections obtained from 3-month-old C57 black mice were included.