Screening of Cre-transfected cells. In A, DNA was digested with PvuII, electrophoresed on agarose gels, transblotted as given in Materials and Methods, and hybridized with the 3′ probe. Lanes 2 to 4 and 6 to 9 yielded a 7.6-kb wild-type signal and a 7.1-kb signal, indicating they have lost both Neo and Trsp. Lane 5 yielded a signal with a slight upward shift of ≈100 bp, indicating loss of Neo. In B, PCR analysis of the fragments in lanes 2 to 9 of panel A with CKNO2 and Spe2 as primers showed a 700-bp fragment in lane 5, confirming the loss of Neo and presence of Trsp only in the DNA of cells shown in lane 5 of panel A. Lane 1 in panel A and lanes 1 and 10 in panel B contain molecular size markers.