Gene inactivation has become the gold standard for determining gene function in the mouse. Many genes inactivated in the germ line cause early lethality that precludes phenotypic assessment at a later time point. Conditional gene inactivation using Cre recombinase expressed via a tissue specific promoter/enhancer allows phenotypic analyses of selected tissues, but lacks temporal control. Recent development of the tamoxifen-inducible Cre-ERT2 offers both cell type-specific and temporal control of conditional gene inactivation. As an example, we describe the design and step-wise construction of aCre-ERT2knock-in allele at thePax7locus using the recombineering method –Pax7is selectively expressed in embryonic muscle progenitors and adult muscle stem cells. The resultingPax7-Cre-ERT2(Pax7CE) allele has been successfully applied to embryos and adults for tamoxifen-regulated myogenic lineage tracing and gene inactivation (Nature 460:627–631, 2009; Genesis 48:424–436, 2010).
