However, it has now been largely replaced by other, simpler measures that are approximations of GFR (7).These measures, which involve clearance of such substrates as EDTA, iohexol, cystatin-C, 125 I-iothalamate, the chromium radioisotope 51Cr (chelated with EDTA), sodium radioiothalamate, and creatinine, have had their utility confirmed in large cohorts of patients with chronic kidney disease (8). The gene discussed is CST3; the disease is chronic kidney disease.