NOTCH1 and intervertebral disk degenerative disorder: The identification of CD24-positive NP cells as the resident progenitor cells/notochordal cells in disc regeneration as well as elucidation of the crucial role of the HIF-1α–NOTCH1 pathway in the phenotypic maintenance of CD24-positive NP cells provides new insights into the beneficial effect of NP progenitor/notochordal cells for the treatment of disc degeneration (Fig. 8).