CDKN2A and amyotrophic lateral sclerosis: Both in ALS animal models and patients, aged astrocytes develop senescence markers such as p16INK4A, p53, p21, and SA-β-gal, becoming toxic for motor neurons (Martin, 2000; Das and Svendsen, 2015; Turnquist et al., 2016), suggesting a causal pathogenic role in mediating motor neuron loss.