VDR and fleck corneal dystrophy: Notably, our group has demonstrated that activation of the nuclear receptor Vitamin D receptor (VDR) by its specific ligand 1,25-dihydroxyvitamin D3 (or calcitriol) resulted in significant increases in RFC expression and activity at the BBB which could effectively restore brain folate concentrations in mice lacking Frα (a model reflective of CFD) [19, 20].