A 2.2 kb region of the gene containing exon 2 was deleted and replaced with a PGK-neo cassette via homologous recombination resulting in deletion of half of the extracellular ligand-binding domain. The targeted mutation removes 3 of 7 conserved cysteine residues required to maintain the conformation of the extracellular domain, as well as 2 of 5 hydrophobic domains. RT-PCR analysis of RNA from homozygous mutant animals confirmed the presence of stable mutant transcript. Sequence analysis of the RT-PCR products confirmed that exon 2 was deleted and that exons 1 and 3 were in-frame.