A targeting vector was constructed with Pdgfra homology arms, the tamoxifen-inducible MerCreMer cDNA with an adenoviral splice acceptor sequence (SA) added to the 5' side of the MCM cDNA to prevent cryptic splicing and an Frt-flanked neo cassette 3' to the the MCM sequence. This construct was injected into ES cells. Flp excised the neo cassette from the final allele. (J:193389)