GFR was measured using iohexol clearance and compared to GFR estimated by the MDRD, creatinine-based CKD-EPI, cystatin-based CKD-EPI, creatinine-cystatin-based CKD-EPI, Berlin Initiative Study (BIS) creatinine, and BIS creatinine-cystatin formulas. The gene discussed is CST4; the disease is chronic kidney disease.