In-frame insertion of a lac Z-neomycin resistance cassette into the second coding exon of the gene, just upstream of the DNA binding domain, eliminated both DNA-binding and ligand-binding functions of the receptor. Whole-mount beta-galactosidase assays revealed conspicuous beta-gal activivity in the interscapular brown fat pad of only E14.5 embryos carrying a targeted allele, but not in wild-type embryos (J:58223)