Deletion of nucleotide 5449 in exon 26 and a loxP-flanked PGK-neo cassette inserted into intron 24 were introduced to the gene via homologous recombination. The single nucleotide deletion should result in a truncated protein similar to the apoB-38.9 mutation found in human familial hypobetalipoproteinemia (FHBL). Western blot analysis of homozygous mutants showed an unexpected mutant protein similar to apoB-27.6, a smaller form of mutant protein found in FHBL. Southern blot analysis of homozygous mutant animals showed the gene was correctly targeted. (J:76047)