CX3CR1 and Abnormal retinal morphology: As a further test of the role of microglia in prion-induced photoreceptor degeneration, we also studied prion-induced retinal disease in mice homozygous for expression of a GFP transgene in place of the Cx3cr1 open reading frame, resulting in knockout of Cx3cr1 expression (tgGFP-Cx3cr1KO) [8].