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1. An expression cassette for expression of a single guide RNA (sgRNA) of a clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system, said sgRNA having a stem of 15 nucleotides or more, said cassette comprises: a promoter, an sgRNA-encoding sequence that is interrupted by a transcription disruption sequence, and a sequence flanked by at least a pair of recombinase recognition sites, wherein said recombinase recognition sites are in the same orientation and wherein recombinase activated recombination at the pair of recombinase recognition sites is capable of excising said flanked sequence, wherein said flanked sequence contains the transcription disruption sequence, wherein the sgRNA-encoding sequence comprises a stem sequence and a loop sequence with a sequence encoding a Cas9 binding element located in the stem sequence, and wherein the sgRNA-encoding sequence that is interrupted by the transcription disruption sequence is capable of expressing the sgRNA through the recombinase activated recombination, and wherein a first site-specific recombinase recognition site of the pair of recombinase recognition sites is located in a sequence that encodes the stem sequence and the loop sequence of the sgRNA sequence and wherein a second site-specific recombinase recognition site of the pair of recombinase recognition sites is located in the sequence that encodes the stem sequence and the loop sequence of the sgRNA sequence.
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