Exon 2 was replaced with one that contained nucleotide substitutions leading to the amino acid substitution of arginning for lysine at position 785. A floxed gene trap vector with a neo cassette and GFP gene was inserted downstream of exon 2. In the absence of cre, a truncated transcript (of 813 amino acids) with the K785R mutation is expressed, but it is quickly degraded and no protein product is detected by western blot analysis. Removal of the gene trap vector by cre-mediated recombination is required to express a full-length product with the K785R mutation.