In support of our notion, mass spectrometry analysis revealed that S130 in p21 is indeed phosphorylated in mitotic HeLa cells [39, 40], although this residue is also phosphorylated by Cdk2/cyclin E in the S phase [14], Cdk6/cyclin K upon viral infection [15] and ERK2 after mitogenic stimuli [24]. Here, CDK6 is linked to viral infectious disease.