VOLUME 284 (2009) PAGES 21057–21065
We recently discovered that some of the loxP sites were either duplicated or inverted. Based upon a complete sequence analysis of the Klf6 conditional allele, we found that the second loxP site had been duplicated, and the third loxP site had been placed in the opposite orientation, thus leading to a Cre-mediated inversion rather than a deletion. (Please see below figure for the desired versus sequence-verified allelic configurations.)
In this configuration, Cre-mediated inversion should still result in a non-functional allele, as a stop codon will be encountered after exon 1; moreover, the truncated N-terminal portion of the protein should be degraded rapidly. In addition, this allele could, in theory, undergo repeated inversion in the presence of long term Cre expression in vivo, thereby toggling back and forth between the functional and non-functional alleles (i.e. on/off states). Although we believe that the Klf6 allele in the mutant mice is non-functional, we also feel it is important that the readers are aware of this new information.
