eGFP expression in RL5 cells carrying different transgene cassettes. (A,B) Cre recombinase-mediated cassette exchange (RMCE) was used to exchange the L1-HYTK-1L cassette at the RL5 locus (chromosome 4) in the MEL cells by pYB cassette containing different repeats. After RMCE, transgenes insert in orientations A or B. eGFP expression was measured in RL5 cells carrying the indicated cassettes in the indicated orientation. Each bar, measurements of three to six independent clones. (C,D) FLP recombinase-mediated excision of DNA sequences. eGFP expression before and after FLP-mediated excision of the vector backbone and repeat sequences. Bars, eGFP expression in an individual clone. p212 cassette is a control.