Three important recent developments may help guide contemporary GFR estimation in cancer: (1) an increase in the use of available validated equations to estimate GFR that are more accurate as compared with measured GFR (mGFR), (2) the availability of validated serum cystatin C-based and combined serum creatinine and cystatin C-based eGFR equations, and (3) studies assessing the performance of newer/updated validated equations (e.g., the 2021 CKD Epidemiology Collaboration [CKD-EPI] equation and the 2023 European Kidney Function Consortium [EKFC] equation). This evidence concerns the gene CST3 and cancer.