An insertion-type vector was used to create mutations in exon 26 in sequences encoding a region in the encoded protein thought to be important for LDL receptor binding by replacing these sequences with beta globin (Hbb-bs) sequences. The beta globin sequence inadvertently contained an in-frame stop codon 5' of the LDL receptor binding domains. Western blot analysis on plasma derived from heterozygous and homozygous mice demostrated that the expression of the ApoB48 isoform is unaffected by this mutation, while a truncated mutant ApoB70 isoform is produced from this allele. (J:16352)