TP53 and Intervertebral disk degeneration: Using NP specimens from patients with degenerative disc disease, Kim et al. demonstrated p53 and p21 expression in senescent NP chondrocytes in all specimens, but p16 was expressed in a few NP chondrocytes in only two specimens, indicating the p53-p21 pathway rather than p16 pathway, plays an important role in NP cell senescence (Kim et al., 2008; Kim et al., 2009).