A similar finding was found in the study by Ayugun B in 23 SCD patients with a mean age of 7.4 years on a mean dose of 24.4 mg/kg/day hydroxyurea for 25 months, the serum cystatin level at baseline and post hydroxyurea therapy was 0.72 ± 0.09 mg/l and 0.74 ± 0.13 mg/l [46]. The gene discussed is CST4; the disease is Schnyder corneal dystrophy.