CDKN1A and Ataxia-telangiectasia: Further studies need to be performed on cyclin-dependent kinase inhibitors, senescence-associated β-galactosidase, the proteins p53, p21cip1, ataxia-telangiectasia mutated protein, ataxia telangiectasia, and Rad3-related protein, and phosphorylated histone H2AX as markers of cellular senescence.