The Cre recombinase-mediated cassette exchange system (RMCE). (A) Cre RMCE was used to exchange the L1-HYTK-1L cassette at the RL5 locus (chromosome 4) in the MEL cells by the p212 or pYB cassettes. Both cassettes have the human beta-globin LCR 234-beta-eGFP and two inverted loxP sites (L1 and 1L). These sites flank the eGFP transgene in p212. In pYB cassette, both eGFP and the YAC/BAC vector backbone sequence are exchanged into the target site. The pYB cassette has two recognition sites for FLP recombinase (FRT). After RMCE, transgenes insert in orientations A or B. (B) Relative eGFP expression is shown in orientation A and orientation B. (C) FLP recombinase-mediated excision of vector DNA sequences. (D) Fluorescence micrograph of MEL cells carrying pYB transgene cassette before or after FLP-mediated excision of the vector backbone sequences.