An X-linked Myh11-CreER mouse line resulting from Y to X chromosome-translocation of the Cre allele.

The Myh11-CreER mouse line (Cre ) has gained increasing application because of its high lineage specificity relative to other Cre drivers targeting smooth muscle cells (SMCs). This Cre allele, however, was initially inserted into the Y chromosome (X/Y ), which excluded its application in female mice. Our group established a Cre colony from male ancestors. Surprisingly, genotype screening identified female carriers that stably transmitted the Cre allele to the following generations. Crossbreeding experiments revealed a pattern of X-linked inheritance for the transgene (k > 1000), indicating that these female carries acquired the Cre allele through a mechanism of Y to X chromosome translocation. Further characterization demonstrated that in hemizygous X/X mice Cre activity was restricted to a subset arterial SMCs, with Cre expression in arteries decreased by 50% compared to X/Y mice. This mosaicism, however, diminished in homozygous X /X mice. In a model of aortic aneurysm induced by a SMC-specific Tgfbr1 deletion, the homozygous X /X Cre driver unmasked the aortic phenotype that is otherwise subclinical when driven by the hemizygous X/X Cre line. In conclusion, the Cre allele carried by this female mouse line is located on the X chromosome and subjected to X-inactivation. The homozygous X /X mice produce uniform Cre activity in arterial SMCs.