PTEN and colorectal carcinoma: miR-21 exerts its oncogenic role by inhibiting the expression of several well-known tumour suppressor genes, such as phosphatase and tensin homolog (PTEN) [195], and programmed cell death 4 (PDCD4) [196], which in turn alter the expression of their downstream targets, leading to the increased invasion, intravasation, and metastasis of CRC cells.