Exon 1 of this gene was replaced with a PGK-neo cassette via homologous recombination resulting in the deletion of the first 121 amino acids, which includes the intracellular N terminus, the first transmembrane domain, and a portion of the extracellular domain. Immunoprecipitation and Western blot analysis failed to detect protein in extracts from brain and hippocampus of homozygous mutant animals.