Cre recombinase mediated cassette exchange. (A) Shown is a diagram of the predicted exchange reaction and the size of the resulting restriction enzyme fragments. (B) Southern blot analysis was performed to verify that the cassette-mediated exchange reaction had occurred. Shown are the results of Southern blot analysis of selected clones of DNA digested with DraIII (top) or BglII (bottom) and probed with the DsRed probe (left) or the LMO2 probe (right). The arrow marks the position of the predicted band with each probe. G is the original LTR-GFP clone, and 24, 25, and 26 are clones in which the LTR-GFP has been excised without insertion of the LTR-DsRed cassette. (C) Western blot analysis of proteins extracted from selected clones containing the LTR-GFP cassette (G-4 and G-25) or the DsRed cassette (R-6, R-23, R-65, and R-75). An untransduced Jurkat cell clone (M-7) and K562 cells served as negative and positive controls, respectively.