Insertion of a neomycin-thymidine kinase expression cassette resulted in a duplication of part of the gene (the second through the fifth coding exons, with the duplicated copies separated by the selectable markers), and inserted into the third coding exon a 21 bp sequence that placed a stop codon in each reading frame. Northern blot analyses failed to detect transcripts from this allele in lung, ovaries or embryonic fibroblasts. (J:43744, J:94836)