Quantitation of mutant loxP recombination following the induction of Cre recombinase. (a) Configuration of the AID locus in an AID -/- DT40 mutant before and after Cre recombinase induction. The cell clone stably expresses a transfected MerCreMer construct. PCR primers for detecting mutant-loxP recombination are shown together with the sizes of the resulting PCR products. (b) Time course of mutant loxP recombination. The cells were cultured for the indicated time in the presence of 0.05 mM 4-hydroxy tamoxifen for inducible-Cre activation. Crude extract of 500 cells were utilized for PCR reaction. The region downstream of loxP_LE, which is not affected by recombination, was amplified as a positive control.