Low muscle mass is the most important non-GFR determinant of serum creatinine in patients with cancer, whereas high adiposity significantly affects cystatin C.21 In the Onco-GFR study, the reported bias for CKD-EPI on the basis of creatinine (CKD-EPIcr), cystatin C (CKD-EPIcys), and CKD-EPIcr-cys for patients with body mass index (BMI) <20 kg/m2 were −16.8 (−21.4 to −13.6), −1.4 (−4.8 to 3.9), and −8.9 (−14.2 to −6.1) ml/min per 1.73 m2, respectively. This evidence concerns the gene CST3 and chronic kidney disease.