The amino and carboxy terminals were targeted in cis to generate an allele from which no protein is produced. The amino terminal was disrupted by the insertion of IRES-GFPLacZ-loxP and an frt-flanked hprt cassette. A floxed neo cassette disrupted the carboxy terminal exon encoding the first transmembrane domain, common to all isoforms. (J:83192, J:151085)