Levels of CDKN1A transcript (encoding the cyclin-dependent kinase inhibitor P21, known to inhibit G1/S progression) in the kidney correlate with worsened renal function in human CKD,3 with elevated P21 (in the absence of proliferation markers such as Ki67) used to identify senescent epithelia in clinical research settings.4 The gene discussed is CDKN1A; the disease is chronic kidney disease.