APOE and age-related macular degeneration: As an example, genetic variants of Cx3cr1 (a microglial homeostatic marker gene that is not expressed by any other cells in the retina), apolipoprotein E2 isoform (APOE), and complement factor H (Y402H) are associated with increased accumulation of microglia and monocytes in the SRS in human AMD patients [11,12,13,14,15].