A cassette containing a stop codon, a SV40 polyadenylation site, and a neomycin resistance gene was inserted immediately downstream of the exon encoding the transmembrane domain (upstream of sequence encoding the cytoplasmic domain). Expression analyses showed the presence of truncated transcript and protein in mutant mice. While Northern blot analysis detected increased levels of mutant transcript, Western blot analysis showed protein levels to be comprable to wild-type. FACS analysis confirmed that mutant protein was correctly expressed on the cell surface.