A vector was designed to replace exons 6 and 7 with an FRT-flanked PGK-neomycin cassette. These exons encode two transmembrane domains and a large portion of its ligand binding domain. Alternative splicing of exon 5 with exon 8 would result in an immediate premature stop codon and should lead to a nonsense-mediated degradation of the mRNA. RT-PCR, Northern blot and in situ hybridization assays were performed to confirm absence of transcript in mutants.