IRBP-mediated NMP expression was detected in rods and cones, but immunohistochemistry revealed that while most cones expressed the transferred gene, some did not. The reason for this variation is not known, but it is possible that cones differentially express the nucleolin cell-surface protein known to mediate uptake of the nanoparticles [12]; alternatively, heterogeneity in the response to the injection insult may affect NMP expression/uptake. In spite of variation in nanoparticle-driven gene expression in cones, we observed essentially complete functional cone recovery (to WT levels) in many IRBP-NMP treated animals. Cone rescue is likely of greater magnitude than rod rescue because cones need less RDS to form fully functional OS than do rods [24]. Furthermore, the RDS haploinsufficiency phenotype is much less severe in cones than in rods, and we have recently demonstrated a differential role for RDS in the two photoreceptor types [47].