Abstract: An expression cassette for conditional expression of a single guide RNA (sgRNA) of a CRISPR/Cas9 system, the cassette includes a promoter, an sgRNA sequence, and a sequence flanked by at least a pair of recombinase recognition sites, wherein recombinase activated re-combination at the pair of recombinase recognition sites is capable of excising said flanked sequence, whereby either i) at least one of said recombinase recognition sites is located within the sgRNA sequence and said flanked sequence contains an transcription disruption sequence or ii) said flanked sequence is at least a part of the promoter or of the sgRNA sequence; methods of conditional expression of sgRNA and a reaction product of the conditional expression, i.e. sgRNA with a recombination site remnant.