In osteoclast, miR‐21 was shown to promote osteoclastogenesis by downregulating PDCD4, which would in turn de‐represses c‐Fos, a key transcription factor for macrophage‐osteoclast differentiation.94 Similarly, a recent study using miR‐21 KO mouse model also supported the role of miR‐21 as an osteoclastogenic promoter.95 MiR‐21‐/‐ mice showed protection against age‐related osteopenia with increased PDCD4 level and trabecular bone mass accrual despite the elevated serum RANKL. This evidence concerns the gene PDCD4 and age.