Schematic representation of the vectors and Cre-mediated recombination. (A) Structure of pBS-DS-hIgG. Before recombination, neither hIgG light chain nor heavy chain was expressed in the transfected cells because of the insertion of two floxed stuffer sequences located at the 5' end of the hIgG genes. (B) Structure of pBS-DS-hIgG after the deletion of two stuffer sequences (ΔDS-hIgG). Following Cre-mediated recombination of pBS-DS-hIgG, loxP- and loxP511-flanked stuffer DNA sequences are removed, and then hIgG light and heavy chain expression is driven by the CAG and CMV promoters, respectively. (C) Structure of pBS-Ova2.8-Cre. The vector consists of a 2.8-kb fragment at the 5' end of the ATG codon of chicken ovalbumin (Ova2.8), an oviduct-specific regulatory sequence, and the Cre recombinase gene. (D) Structure of the chicken oviduct-specific hIgG expression vector pBS-Ova2.8-hIgG. In pBS-Ova2.8-hIgG, expression of the hIgG light and heavy chains is individually promoted by the ovalbumin promoter Ova2.8.