Exon 4 was disrupted by the insertion of a neomycin resistance gene. Exon 4 encodes part of the extracellular domain. Northern blot revealed a transcript shorter than wild-type. 5' RACE experiments revealed that the start site for this short transcript was 23bp downstream of the neo in exon 4. Western blot analysis using antibodies raised against the intracellular domain indicated that the allele is a null.