Exons 358 (MEx1) and 359 (MEx2) were left flanked by single loxP sites. A single frt site remained immediately upstream of the 3' loxP site, but transcription was reported to be unaffected by the presence of the frt and loxP sites. Recombination of the floxed region, which encodes the kinase domain, results in an in frame union of exons 357 and 360.