Fig. 2. PCR analysis and Southern blot. (A) PCR analysis of genomic DNA used to initially screen for targeted cells. The gel shows duplicate reactions for 4 separate clones (16, 20, 25 and 68-G) that were the only clones to give positive bands by this PCR screen. Clone 16 (lanes 1and 2), clone 20 (lanes 3 and 4), clone 25 (lanes 5 and 6), clone 68-G (lanes 7 and 8) and lane 9 is a negative control. (B) Subsequent southern blot for 3 of clone 20 (lane 1), clone 25 (lane 2), clone 68-G (lane 3) and wild-type (lane 4) cells. The blot was probed using a genomic DNA probe that was outside the targeted region (see Fig. 1). Clone 20 is the only clone that shows a targeted band as well as a wild-type band, with no other additional bands. We expected that this probe would detect a band of 5.7 kb in an untargeted allele, and that the size of this band would increase to 7.2 kb, if the allele had been targeted.