The exon after exon 11 (i.e. exon T1) is used in vivo to produce a dominant negative form of the gene product that has extracellular and transmembrane domains but lacks the intracellular signaling domain. A loxP site was inserted into the BamHI site upstream of exon T1 and a loxP- and frt- flanked neomycin cassette was inserted into BamHI site downstream of exon T1. Cre-mediated recombination was used in correctly targeted mice to remove the T1 exon and the neomycin cassette. No T1 isoform was detected by immunoblot assays of homozygote brain extracts while expression of the full length protein was unaffected.