Indeed, we found that CM-BC/circIKBKB treatment induced the expression of multiple osteoclastogenesis-related markers, including FBJ osteosarcoma oncogene (C-fos), acid phosphatase 5, tartrate resistant (Acp5), cathepsin K (Ctsk), nuclear factor of activated T cells 1 (Nfat-c1) and dendrocyte expressed seven transmembrane protein (Dc-stamp), and facilitated the fusion of pre-osteoclasts, accompanying with increased podosome (actin ring) formation (Fig. S2d and Fig. 2d). Here, CTSK is linked to breast cancer.