In the post hoc analysis (Tukey test) cystatin C levels were significantly lower (p < 0.0016) in the control (0.70 ± 0.13 mg/L), HNF1A-MODY (0.75 ± 0.21 mg/L), and GCK-MODY (0.2 ± 0.16 mg/L) groups in comparison to those with either T1DM (0.87 ± 0.15 mg/L) or T2DM (0.9 ± 0.23 mg/L). The gene discussed is GCK; the disease is type 2 diabetes mellitus.