A mouse BAC clone containing the Nkx2-1 gene was engineered to express cre recombinase. Fifteen base pairs at the 5'end of exon 2 of the Nkx2-1 gene were replaced with a cre-poly(A) construct. The mutated BAC clone was then used to generate transgenic mice. This construct is expressed in brain, thyroid, pituitary, and lung, with telecephalic expression first detectable at E10.5.