Fig. 2. Sister chromatid recombination within the SCneo locus is not associated with reciprocal exchange. After I-SceI expression in cell lines containing the SCneo substrate, single cells were grown in non-selective medium and then replica plated to test for resistance to G418. Southern blot analysis was performed on clones containing G418R cells using XhoI and HindIII digestion of genomic DNA. The probe was a 1.2 kb BamHI–XhoI DNA fragment containing the complete neo sequence (Figure 1A). (A) Parental V79 (4-18) cells and clones in which G418R cells were identified. STGC events, lanes 1–4, 6, 8, 9, 12, 14 and 15; LTGC events, lanes 5, 7, 10, 11 and 13. (B) Parental V79 (4-18) cells and clones in which cells were identified to have undergone an LTGC event (lanes 1–12) or an HD event (lane 13). In both (A) and (B), each of the expansion products is presumed to be derived from an LTGC event, as none of the clones contains an associated 0.7 kb reciprocal SCE product. M, marker derived from V79 (4-18) genomic DNA digested with XhoI, BamHI and HindIII.