Polgtm1Lrsn mice were mated with ubiquitously expressing Flp-mice to excise the Frt-flanked neo, leaving exon 3 flanked with loxP sites and an aspartic acid to alanine codon change at 257, in the second exonuclease domain. Biochemical assays revealed a profound reduction in exonuclease activity in these mutants. (J:91077)