A loxP-flanked neomycin and thymidine kinase selection cassette was inserted into exon 11. The loxP flanked cassette was removed in ES cells by Cre mediated recombination prior to the production of chimeric mice, leaving a 50 bp insertion containing a loxP site. Northern blot analysis on RNA derived from total RNA of homozygous embryos demonstrated that aberrant transcripts were produced from this allele that either lacked exon 11, or contained exon 11 with the insertion, resulting in a frameshift mutation after the first 924 amino acids. This mutation was made to mimic a truncated protein associated with human breast cancer.