Fluorescence revertance assay to monitor AID activity. (A) Representation of the pMX-PIE-mOrange^STOP and AID-ER-ΔhuCD4 or AID^E58Q-ER-ΔhuCD4 retroviral vectors used to transduce NIH-3T3 cells. (B) Detection of AID activity. NIH-3T3 cells were co-transduced with the retroviral vectors depicted in A and cultured with (bottom) or without (top) 1 µM OHT. mOrange⁺ cells were monitored by flow cytometry. Representative FACS analyses at day 9 are shown. (C) Time-course analysis of mOrange⁺ cell appearance in NIH-3T3 cells co-transduced as described in B. One representative experiment is shown.