A neomycin resistance gene replaced exon 16, encoding the transmembrane domain. No clear bands were seen for exons downstream of exon 15 in RT-PCR analysis, showing that the protein was truncated at the termination codon in exon 15. Mutant lung tissues demonstrated that the 110-kDa endogenous soluble form was present, as it results from a termination in intron 13. Also present was a new truncated form, approximately 120-kDa, detected with an antiserum raised against the amino acid sequence of exon 15.