Fig. 3

New breeding techniques used for GM crops development. Zinc finger nucleases (ZFN), transcription activator-like effector nucleases (TALEN) and the bacterial system of clustered regularly interspaced short palindromic repeats (CRISPR), employing Fok1 or Cas9 nucleases, are used to target DNA sequences to promote downstream modifications. ZFN, TALEN and Cas9 induce double-strand breaks that are corrected by NHEJ or HDR, modifying the target sequence with deletions or different size insertions. Modified Cas9, such as catalytically null (“dead” Cas9 or dCas9) is used coupled to transcriptional repressor or activators to regulate gene expression. Other forms of modified Cas9, such as coupled to reverse transcriptase (RT) or deaminases, are used to modify target sequence with specific template primers (prime editing) or switch specific bases (base editing).