A neomycin selection cassette replaced a 2.3-kb fragment containing exons 8 through 11. The neo gene was situated in the reverse transcriptional orientation and was excised during the splicing of the transcript, resulting in a frameshift that introduces a stop codon in exon 12. A truncated transcript was observed by RT-PCR analysis. A 90% decrease in binding of a radio-labeled ligand in homozygous mutants was observed versus wild-type, as well as an absence of cAMP production stimulation.