An FRT stop cassette containing the mEGFP(A206K) coding sequence separates UAS regulatory sequences from a bicistronic sequence that encodes two open reading frames separated by stop codons; a 114bp upstream ORF (u[M]) and a downstream ORF that encodes human codon-optimized Cas9. In the absence of FLP recombinase, mEGFP(A206K), but not u[M] or Cas9, is expressed.