A floxed neo cassette was inserted into the 5' untranslated region and an additional loxP site was inserted downstream of exon 1. Cre-mediated recombination by transient expression of cre deleted the exon 1. which contains the initiation methionine and a transmembrane domain. Without the signal peptide and the transmembrane domain no functional protein is made. The lack of transcript was confirmed by RT-PCR.