Figure 3.—

Representative Southern blot analysis of G418^R clones. Genomic DNA samples (8 μg) isolated from eight G418^R clones recovered from cell line CB6 were digested with BamHI and displayed on a blot using a tk-specific probe (lanes 1–8). DNA samples from the clones shown in lanes 3, 4, and 8 were additionally digested with BamHI plus I-SceI and displayed in lanes 9, 10, and 11, respectively. Lane 12 displays parental cell line DNA digested with BamHI plus I-SceI. As discussed in the text, the clones in lanes 2 and 5 produced a BamHI fragment notably shorter than 3.9 kb, indicative of relatively large deletions at the I-SceI site, and the clones displayed in lanes 3, 4, and 8 (and lanes 9–11) had undergone an apparent duplication of the integrated tk-neo gene. The origins of the fragments visualized are illustrated in Figure 1.