A W196R mutation was introduced into exon 7 by homolgous recombination. A loxP flanked neomycin resistance gene containing the SV40 polyA sequence was inserted into intron 5. Western blot demonstrated that mutant cells express half of the level of protein as that of wild-type cells. Cre-mediated recombination restores the expression of the mutant transcript containing the point mutation, and expression of the mutant protein.