Additionally, miR-21 derived from MDA-MB-231 and SCP28 exosomes was found to be highly expressed in serum exosomes of breast cancer patients with bone metastasis, which altered the protein level of programmed cell death 4 (PDCD4), thus contributing to the generation of pre-metastatic niches, the promotion of osteoclast differentiation, and the enhancement of bone metastasis [7]. Here, PDCD4 is linked to bone metastasis.