A 12.3 kb region of genomic sequence starting with the 5' UTR and including all coding exons as well as part of the 3' UTR were replaced with a floxed neomycin resistance gene. Transient transfection with a cre expressing plasmid then eliminated the neomycin gene. Absence of mRNA transcripts in mice derived from these ES cells was demonstrated by reverse transcription and PCR. (J:85497)