For instance, in the context of intervertebral disc degeneration, up-regulation of miR-21 could promote proliferation of human degenerated nucleus pulposus cells through regulating PDCD4 expression, enhancing phosphorylation of c-Jun protein, and activating AP-1-dependent transcription of MMP-2/9 (29). This evidence concerns the gene JUN and Intervertebral disk degeneration.