ES cells were re-targeted with an "F2A-CreERT2" vector and an FLP-expressing plasmid to facilitate recombination. Correctly re-targeted ES cells had (from 5'to3') an frt3 site within Trib 2 intron 1, a self-cleaving 2A peptide sequence designed to splice a cre/ERT2 fusion gene into a frame with the Trib 2 exon 1 sequence, a bovine growth hormone polyA signal, an AttB site, a PGK-hygromycin-SV40polyA cassette (with a splice donor-frt5 site-RNA splice acceptor in the hygromycin gene), an AttP site, and Trib 2 exon 2 deleted. The resulting chimeric animals were bred to PhiC31-expressing mice to delete the AttB/AttP-flanked sequences and replace them with the recombined AttB/AttP site (AttL). (J:199218, J:220523)