PDCD4 and cancer: miR-21 is one of the most prominent onco-miRs in most cancer types owing to its suppression of a network of bona fide tumor suppressor (TS) genes, including Reversion-inducing cysteine-rich protein with Kazal motifs (RECK), Tropomyosin 1 (TPM1), Phosphatase and tensin homolog (PTEN), Programmed Cell Death 4 (PDCD4) [11], Grainyhead-like transcription factor 3 (GRHL3) [12] and others.