For instance, miR-21, which is highly expressed at tumor cells and promotes tumor growth by inhibiting the expression of dimethylarginine dimethylaminohydrolase 1 (DDAH1), phosphatase and tension homology deleted on chromosome ten (PTEN), and programmed cell death protein 4 (PDCD4) [36,37,38]. The gene discussed is PDCD4; the disease is neoplasm.