Sequence encoding the DNA binding and dimerization domains of the protein was replaced by the insertion of an NLS-betagalactosidase-neomycin cassette. In situ hybridization demonstrated expression in embryos ranging from E7.5 to E9.5 predominantly in the cardiac crescent and later in the heart tube, the somites, and the precondensed mesenchyme in the tail, consistent with the expression of the endogenous gene.