In a study conducted by Alvarez o et al. among 193 SCD patients with a mean age of 13.8 months on 20 mg/kg/day hydroxyurea, the serum cystatin level at baseline and 24 months after hydroxyurea therapy were 0.91 ± 0.17 mg/l and 0.92 ± 0.13 mg/l respectively [44]. Here, CST4 is linked to Schnyder corneal dystrophy.