PTEN and non-small cell lung carcinoma: miR-21 stimulates cell growth and NSCLC cell invasion by targeting PTEN (phosphatase and tensin homolog), which enhances the RAS/MEK/ERK pathway by repressing negative regulators of the pathway (i.e., Spry1, Spry2, Btg2, and Pdcd4) and by repressing the expression of proapoptotic proteins (e.g., Apaf1, FasL, RhoB, and Pdcd4).