Exon 3 was flanked by a floxed neo cassette inserted into intron 2 and a single loxP site in intron 3. Several aberrant transcripts involving the splicing of exon 2 to cryptic slice sites in the neo transgene were identified by RT-PCR and sequence analyses. Further analysis indicated that the readthrough of the neo transgene will result in functional ablation due to truncation or the inclusion of a random neo fragment. Mice carrying this allele were crossed with transgenic mice expressing cre recombinase via the EIIa promoter to generate Igf1rtm1.1Mhz and Igf1rtm1.2Mhz.