e to TK loss and analyzed their DNA by PCR and Southern blot. Strikingly, 54 proper recombinants were identified, indicating very high RMCE efficiency (83%). RMCE also proved to be precise, as no aberrant bands were detected in PCR and Southern blots (Figure 3A). p53+/GFP ES clones were analyzed by western blot with an antibody against p53, and found to express an addit