A positive feedback loop of c-Fos/miR-21/PDCD4 is critical during stimulation of osteoclastogenesis by RANKL. c-Fos triggers the miR-21 gene, resulting in down-regulation of PDCD4 protein levels, which in turn further stimulates c-Fos transactivation, which leads to NFATc1 expression and BMMs differentiating into osteoclasts.