Tryptophan codon 298 (TGG) in exon 2 was changed to alanine (GCG) (p.W298A) by RMCE using (ES) cells containing the Ptf1atm1Mgn allele. The exchange cassette contained the Ptf1a gene SalI-SalI fragment from strain 129 genomic DNA with the codon 298 mutation.