miR-21 upregulation has been found to downregulate PTEN expression during gefitinib treatment, which led to the activation of the phosphoinositide 3-kinase/protein kinase B and mitogen-activated protein kinase/extracellular signal-regulated kinase signaling pathways, known pathways involved in gefitinib resistance in NSCLC tumor cells [101]. This evidence concerns the gene PTEN and non-small cell lung carcinoma.