The biogenesis of miRNAs, including let-7, is regulated at different stages in the nucleus and cytoplasm by miRNA processing proteins, such as Drosha and Dicer, respectively, and shRNA-mediated downregulation of Drosha or Dicer1-enhanced tumor development in a K-Ras-induced mouse model of lung cancer, suggesting that the abrogation of global miRNA processing promotes tumorigenesis [40]. This evidence concerns the gene DICER1 and lung cancer.