CX3CR1 and retinal degeneration: To exclude a potential genetic background effect that might contribute to some of the observed variability between different chemokine knockout lines and to understand whether and how combined defects in both chemokine pathways in CCDKO mice contribute to retinal degeneration, we generated Ccl2−/−, Cx3cr1−/− and Ccl2−/−/Cx3cr1−/− mouse lines by backcrossing the original parental line with C57BL/6 mice (figure 6A).