In the early stage of diabetes, there is an increase in pro-inflammatory proteins, such as intercellular adhesion molecule-1 (ICAM-1), inducible nitric oxide synthase (iNOS) and vascular endothelial growth factor (VEGF), that leads to an increased leukocyte adhesion to retinal vessels and a consequent blood–retinal barrier (BRB) breakdown [47,48,49]. This evidence concerns the gene NOS2 and diabetes mellitus.