The first 19 bp of exon 1 were fused in-frame to a lacZ-PGK-neo construct and introduced to the gene via homologous recombination. Northern blot analysis of lung RNA from homozygous mutants using primers to exons 8 and 9 revealed a near complete loss of gene transcription. RT-PCR analysis using primers to exon 2 detected weak expression in lungs from homozygous mutants leading the investigators to believe there may be an additional promoter downstream of exon 1. The lacZ fusion product is expressed properly in the brain as demonstrated by in situ hybridization. (J:59458)