CXCL12 and intervertebral disk degenerative disorder: Fourth, miR-31 has been documented to target several genes in degenerative NPCs, such as MMP3, SDF-1, CXCR7, and ATF6, thus participating in the IDD progression [13, 38, 42], and we thereby cannot exclude the involvement of these targets in the alleviation of miR-31 in BMSC-EVs in the IDD progression due to the complex microenvironment.