The genes P53 (tumor protein P53), P16 (cyclin dependent kinase inhibitor 2A, CDKN2A/P16), and P21 (cyclin dependent kinase inhibitor 1A, CDKN1A/P21) are key cell cycle regulators and are increasingly recognized for their role in aging, senescence, oxidative stress, inflammation, and neurodegenerative diseases including MS [27–33]. Here, CDKN1A is linked to neurodegenerative disease.