MMP9 and intervertebral disk degenerative disorder: In conclusion, miR-21 could promote the proliferation of human degenerated NP cells byregulating PDCD4 expressions, increasing phosphorylation of c-Jun protein, andactivating AP-1-dependent transcription of MMP-2 and MMP-9, suggesting that miR-21 maybe a crucial biomarker in the pathogenesis of IDD.