Methods and systems for targeted genomic modification within a target genome region (TGR) in a mammalian cell. In particular, there is provided a CRISPR/Cas9 system comprising: one or more guide RNA (gRNA) comprising a CRISPR RNA (crRNA) and a trans-activating crRNA (tracrRNA) linked together, the gRNA binding with sequence specificity to a target DNA sequence in the TGR that is adjacent to a PAM sequence； a Cas9n protein； and an optional repair template for homology-directed repair (HDR). The mammalian cell may be contacted with the CRISPR/Cas9 system such that the TGR is modified, forming a modified-TGR, the one or more gRNA and/or the optional repair template selected such that the modified-TGR cannot be further modified by the CRISPR/Cas9 system. A third gRNA may be selected such that the CRISPR/Cas9 system can only bind and/or modify the third target DNA sequence if the TGR comprises a disease-causing modification.
