CXCR4 and osteoporosis: Among those, a few studies have reported on the genetically engineered exosomes that overexpress the C-X-C chemokine receptor type 4 or CXCR4 (CXCR4-Exo) for the treatment of ulcerative colitis, tumors, and osteoporosis [34–36], demonstrating their “homing” potential towards inflammatory sites that highly express the CXCR4 ligand stromal cell-derived factor-1 (SDF-1/CXCL12).