Although serum follistatin levels have been extensively investigated in various diseases, including muscle physiology [27], metabolic disorders [28,29,30], cancer [31,32,33], and cardiovascular diseases [34,35], to the best of our knowledge, the present study represents the first demonstration that urinary follistatin is present in AKI patients, identifying it as a novel biomarker for detecting renal diseases. This evidence concerns the gene FST and cancer.