For example, miR-21 is often viewed as oncogenic in the context of GBM due to its ability to down regulate various tumor suppressor genes, including PTEN, metalloproteinase inhibitor 3 (TIMP3), reversion-inducing cysteine-rich protein with Kazal motifs (RECK) [45], and programmed cell death protein 4 (PDCD4) among others [15,181,182,183]. Here, PTEN is linked to glioblastoma.