Using CRISPR/Cas9 technology, a a nuclear localization sequence (NLS) followed by a codon optimized Cre recombinase fused to ERT2, a GSG linker and a P2A peptide (for production of a bicistronic mRNA transcript) immediately was inserted into the endogenous Myh11 gene immediately after the first codon ATG in exon 2. Cre expression is constitutive rather than inducible by tamoxifen.