ES cells containing Cbfbtm1Ppl were targeted a second time to insert a floxed cassette encompassing exons 5 and 6, as well as the bovine growth hormone polyA signal and a PGK-hygro cassette into the Msc1 site in intron 4. This design allows the wild-type Cbfb transcript to be expressed. Cre expression would remove the cassette and restore expression of the knockin allele.