Insertion of an IRES-lacZ-neomycin resistance cassette into exon 2, which encodes the third helix of the homeodomain, introduced premature translation stop codons in all three open reading frames. This allele produces beta-galactosidase and neomycin phosphotransferase under the control of the endogenous promoter. The authors predict that this allele would also produce a truncated protein that lacks the carboxy terminus. RNase protection analysis did not detect wild-type homeobox in homozygous mutant embryos. (J:73069)