Correlation of transgene expression with viral vector concentration. Mouse eyes were injected with 1 μL of a CMV LacZ vector and enucleated at 2 weeks and processed for X-gal staining (B-D). Serial sections were cut at 6 μm and tissues were counterstained with neutral red to reveal retinal layers (F-H). Controls with no vector injection (A, E, I) reveal no sign of β-galactosidase activity. Eyes injected at 1 × 107 vp/μL (B, F, J) showed transgene expression to be present, but sporadic in the retina and confined to the RPE (M). At a higher dose of 1 × 108 vp/μL (C, G, K) expression was seen around the entire retinal layer and detected in the RPE predominantly (N). At 1 × 109 vp/μL (D, H, L) significant expression was detected around the entire retinal layer with staining in both the RPE layer as well as PR segments (O).