BMI1 and intervertebral disk degenerative disorder: To assess the effect of Bmi‐1 deficiency on IDD and potential rescuing effect by antioxidant NAC treatment, the intervertebral discs of 4‐week‐old mice of the WT, Bmi‐1−/− and Bmi‐1−/−+NAC groups were analysed by immunohistochemical analysis, Western blot and real‐time RT‐PCR.