HDAC9 and hyperhomocysteinemia: To study Hyperhomocysteinemia (HHcy)-induced epigenetic modifications as potential mechanisms of blood retinal barrier (BRB) dysfunction, retinas isolated from three- week-old mice with elevated level of Homocysteine (Hcy) due to lack of the enzyme cystathionine β-synthase (cbs–/–, cbs+/– and cbs+/+), human retinal endothelial cells (HRECs), and human retinal pigmented epithelial cells (ARPE-19) treated with or without Hcy were evaluated for (1) histone deacetylases (HDAC), (2) DNA methylation (DNMT), and (3) miRNA analysis.