An insertion-type vector was used to create mutations in exon 26 at both codon 2153 to alter this residue from an glutamine to one encoding leucine (Q2153L) and at codon 3798 to alter this residue from a leucine to a stop codon (L3798X). Western blot analysis on plasma derived from hemizygous mice demonstrated that a mutant, truncated protein is expressed from this allele, and the expression of the ApoB48 and ApoB100 isoforms was abolished.