PDCD4 and cancer: Further analysis of the tumors implicated a variety of signaling pathways in which the miR-21 plays a pivotal role in the carcinogenesis of many types of cancers [7-11], and some of the signaling molecules have been experimentally validated as targets of miR-21, including phosphatase and tensin homolog (PTEN) [10,12], programmed cell death 4 (PDCD4) [13], reversion-inducing -cysteine-rich protein with kazal motif (RECK) [14], and tropomyosin alpha-1 chain (TPM1) [11].