NGF and non-proliferative diabetic retinopathy: For example, levels of proteins such as nerve growth factor, APOA1, lipocalin 1, lactoferrin, and lysozyme C are significantly elevated in the tears of patients with PDR, whereas in the tears of patients with non-proliferative diabetic retinopathy (NPDR), there are reduced levels of lipocalin 1, hsp27, and β2-microglobulin and elevated levels of endothelin and neuron-specific enolase.