A targeting vector was designed to insert a CreERT2 fusion gene , an SV40 polyA signal, and an frt-flanked neo cassette into the initiation codon of the NK2 homeobox 1 locus (Nkx2-1). This construct was electroporated into embryonic stem (ES) cells. Correctly targeted ES cells were injected into recipient blastocysts and chimeric mice were bred with C57BL/6 mice to originate the colony. Mutant mice were bred with Actin-FLPe mice (on a C57BL/6 congenic background) to remove the neo selection cassette. These Nkx2.1-CreER (or Nkx2.1-CreERT2) mice were subsequently bred to C57BL/6 inbred mice for a few additional generations (and the FLPe transgene was removed).