A targeting vector was designed to place an internal ribosome entry site (IRES) fused to a CreERTM fusion gene (CreERTM; Cre recombinase fused to a G525R mutant form of the mouse estrogen receptor ligand binding domain) downstream of the stop codon in exon 3 of the Myf5 gene. A neomycin resistance marker, flanked by frt sites, was inserted downstream of the IRES-CreERTM cassette. This construct was electroporated into R1 embryonic stem cells. Correctly targeted ES cells were injected into C57BL/6 blastocysts and chimeric males were bred with C57BL/6 females. To remove the neo marker, mice were bred to B6.129S4-Gt(ROSA)26Sor/RainJ mice. (J:199482)