SOX9 and infection: By GFP labeling, some of the cells with high GFP expression exhibited a MG-like morphology and co-expressed Sox9 but not the mature RGC marker Rbpms (Figures 4E–J,N,O), whereas in retinas without AAV infection, GFP, as a RGC-specific marker expressed from the endogenous Brn3b gene locus, was never found in the MG (Figure 4D).