Exon 1, which encodes the first 30 amino acids, and part of the first intron were replaced by a lacZ-PGK-neomycin resistance cassette. The lacZ-coding sequence was placed immediately downstream of and in frame with the initiating codon. In addition, a frameshift mutation was introduced into exon 3. Western blot analysis did not detect protein in heads of homozygous mutant E14.5 embryos. Immunohistochemistry did not detect protein in the brachial region of homozygous mutant E11.5 embryos.