Changes in the lens proteins (crystallins) affect how the lens refracts light and reduce its clarity, therefore decreasing visual acuity. Chemical modification of these lens proteins leads to the change in lens colour. New cortical fibres are produced concentrically and lead to thickening and hardening of the lens in nuclear sclerosis, which often appears yellow and can increase the focusing power of the natural lens. Increasing myopia can also be evidence of a progressing nuclear sclerotic cataract. In an experimental model, oxidative stress contributed to cataract formation, causing a decrease in the level of adenosine triphosphate and glutathione disulfide