Exon 3, which encodes the ligand binding extracellular receptor domain 1 (D1), was disrupted by a neomycin selection cassette inserted by homologous recombination. Two aberrant transcripts were identified in homozygous mutant mice by Northern blot analysis. Both transcripts lacked sequence encoding the extracellular binding domain and were not expected to bind the endogenous ligand. This targeting vector was also electroporated into ES cells derived from a C57BL/6 background to generated Pigrtm1Rast.