A lacZ/neo cassette was inserted in frame after the third codon in exon 1, replacing the remainder of exon 1. Southern blot confirmed recombination in mutant mice. Northern blot indicated a lack of full-length transcript in homozygotes, but presence of a lacZ-containing transcript in homozygotes and heterozygotes. Absence of wild-type protein was demonstrated by an immunohistochemistry assay of sciatic nerves and brain from mutants. (J:92517)