miR-21 mainly causes tumor development, growth, and metastasis by downregulating the genes related to apoptosis, such as phosphatase and tensin homolog (PTEN) and programmed cell death protein 4 (PDCD4), as well as tumor growth and metastasis inhibitory genes, such as reversion-inducing cysteine-rich protein with Kazal motifs (RECK) and tropomyosin 1 (TPM1) [52,53]. Here, PDCD4 is linked to neoplasm.