A neomycin resistance cassette replaced a genomic fragment containing exons D1 and part of D2A. These exons encode the first Ig domain and part of the second Ig domain. Northern blot analysis revealed that a truncated mRNA was produced from this allele, but equilibrium binding studies demonstrated that no protein was present on the cell surface of primary mouse embryo fibroblasts (PMEFs) derived from homozygous mice.