In both Oxford subject groups, we observed similar large differences between cystatin C-based and creatinine-based GFR estimates: the cystatin C-based measure was 27.4 ml/min/1.73 m2 (95% CI: 19.9–34.9; p < 0.0001) higher in HNF1A-MODY and 25.8 ml/min/1.73 m2 (95% CI: 18.2–33.3; p < 0.0001) higher in T2DM. Here, CST3 is linked to type 2 diabetes mellitus.