A loxP site was inserted upstream of 3' enhancer (E3' or Rr159), and a floxed neo cassette was inserted downstream of the E3'. An FRT site was inserted upstream of the downstream enhancer (Ed or Rr160), and an FRT-flanked puro cassette was inserted downstream of the Ed. Cre-mediated recombination removed the E3' and flp-mediated recombination removed the Ed. (J:167472)