The targeting vector contained, from 5' to 3', an internal ribosome entry site sequence (IRES; allows translation initiation in the middle of an mRNA sequence), the Cre recombinase sequence, a bovine growth hormone polyA sequence and a frt-flanked PGK-Neo-bGHpA cassette. Chimeric mice were bred to FLPe mice for germline removal of the PGK-Neo-bGHpA cassette. (J:269094)