PDCD4 and breast cancer: It has been shown that miR-21 overexpression promoted BC progression and chemoresistance via TGF-β/miR-21/phosphatase and tensin homolog (PTEN) signaling axis suppression [25]; miR-21 also targeted tissue inhibitor of matrix metalloproteinases 3 (TIMP3), programmed cell death receptor 4 (PDCD4), tropomyosin 1 (TPM1), and reversion-inducing cysteine-rich protein with kazal motifs (RECK) mRNA, therefore, enhanced cell invasion, tumor metastasis, and angiogenesis [26].