Targeted Rearrangement of Floxed Alleles in Smooth Muscle Cells in Vivo

To the Editor: In their biologically and technically important article “Development of a Smooth Muscle-targeted Cre Recombinase Mouse Reveals Novel Insights Regarding Smooth Muscle Myosin Heavy Chain Promoter Regulation”,1 Regan et al reported generation of mice that express cre recombinase from a fragment of the smooth muscle myosin heavy chain (SMMHC) promoter. The biological importance of this article was that it provided strong evidence of either temporally or spatially restricted expression of the SMMHC promoter in smooth muscle cells in vivo. The paper also had two important technical aspects. First, it demonstrated that expression of a SMMHC promoter-cre construct in cre-indicator mice was a more sensitive means of detecting promoter activity than a construct in which the SMMHC promoter expressed lacZ. Second, the SMMHC-cre mice themselves were said to “provide a powerful tool to researchers to study gene function in vascular development/disease by using cre/lox technology to direct smooth-muscle-specific gene activation or inactivation.” Although the SMMHC-cre mice reported by Regan et al have been used by one group for gene activation,2,3 we—as well as other groups that shared their results with us4 …