CTSS and chronic kidney disease: Furthermore, Cys C, a sensitive biomarker for CKD, has also been found in the retinal pigment epithelium.[30] Several studies have indicated that Cys C is associated with age-related macular degeneration by inhibiting cathepsins and is an independent risk factor for DR.[31, 32, 33] In the current study, we also found that serum Cys C concentration was higher in patients with DR and increased in the fifth year of DR.