There was no statistically significant difference in the GFR estimates in the HNF4A-MODY (p = 0.5), GCK-MODY (p = 0.5) and T2DM (p = 0.8) groups, while in the HNF1B-MODY subjects, the cystatin C-based GFR estimate was lower (p = 0.0009), consistent with the significantly higher cystatin C concentration observed in this subgroup. Here, GCK is linked to type 2 diabetes mellitus.