In T1DM, the CKD-EPI estimate was 12.5 ml/min/1.73 m2 higher (95% CI: 7.3–17.7, p < 0.0001), while in the HNF1A-MODY group the difference was in the opposite direction: the CKD-EPI estimate was 13.8 ml/min/1.73 m2 less than cystatin C-based GFR (95% CI: 8.3–19.3, p < 0.0001). The gene discussed is CST4; the disease is type 1 diabetes mellitus.