CDKN1A and chronic kidney disease: Many cellular senescence markers are upregulated in CKD, such as the cyclin-dependent kinase inhibitors including p21cip1 (p21) 9, 10 and p16Ink4a (p16) 10, 11, phosphorylation of the Ser-139 residue of the histone variant H2AX (γ-H2AX) 12, 13 and senescence-associated β-galactosidase (SA-β-GAL) 9, 11, 14.