Exon 15, which encodes 22 amino acids and the C-terminally located redox center consisting of Gly-Cys-Sec-Gly, and 1.7 kb of the 3' UTR containing the selenocysteine insertion element, essential for cotranslational Sec incorporation at the UGA codon, AU-rich mRNA instability elements and the endogenous transcription stop signal, were deleted from Txnrd1tm1Marc via cre-mediated recombination. RT-PCR confirmed the excision of exon 15 in embryonic mRNAs from E9.5