CRB1 and retinal degeneration: The genetic proof of a third autosomal recessive locus by breeding, genotyping and phenotyping of the lines as well as the presence of Crb1-RD8 mutation in all affected lines, allowed us to subsequently use these lines to assess the differential modulatory effect of the genetic background and the two chemokine loci, Ccl2 and Cx3cr1, on the phenotype of the RD8 retinal degeneration in offspring from the F2 generation.