Human RPE cells (D407 cell line) were obtained from the American Tissue Culture Collection (Manassas, VA) and were cultured in DMEM with 10% fetal bovine serum (FBS; Invitrogen-Gibco, Grand Island, NY), 100 U/mL penicillin, 100 μg/mL streptomycin at 37°C under 5% CO2, and 95% humidified air. The transfection reagent (HiPerfect) was purchased from Qiagen (Hilden, Germany). A first-strand cDNA synthesis kit and real-time SYBR green RT-PCR mastermix were purchased from Toyobo (Osaka, Japan). For Western blot analysis, anti-Robo1 and anti-cyclin D1 antibodies were purchased from Abcam (Cambridge, UK), and an HRP-goat anti-rabbit secondary antibody was obtained from Rockland Immunochemical, Inc. (Gilbertsville, PA) at a dilution of 1:4000. For immunochemistry, the Robo1 antibody was used at a dilution of 1:100. Monoclonal antibodies specific for human CD31 (1:100 dilution), glial fibrillary acidic protein (GFAP, 1:100 dilution), and pan-cytokeratin (1:100 dilution) were purchased from Chemicon (Temecula, CA). Rhodamine phalloidin (Cat. No. PHDR1) was purchased from Cytoskeleton (Denver, CO). Goat anti-mouse FITC and mouse anti-rabbit FITC-conjugated secondary antibodies were obtained from Sigma-Aldrich (St. Louis, MO) and used at a dilution of 1:100. A nuclear staining kit for detecting the distribution of cells in the various cell cycle phases (Cycletest Plus DNA Reagent Kit) was obtained from BD Biosciences (Bedford, MA). All other reagents were purchased from Sigma-Aldrich.