5 CONCLUSION
In summary, it is not desirable to disrupt the synthesis of amino acids or nucleotides in order to construct a heterologous metabolic pathway in metabolic engineering. Currently, the optimized CRISPR-Cas9 gene editing tool may provide an alternative solution to achieve the recovery and reuse of screening markers [16]. Meanwhile, numerous neutral loci in S. cerevisiae have been characterized for multiple gene integration [30]. The recovery of auxotrophic markers to generate the prototrophic strain is supposed to be essential to maximize bio-productions during the fermentation, and alleviate the interference of internal environment for the engineered strains.