Recombination-mediated cassette exchange (RMCE) was used to replace exons 76 through 89 with an frt-flanked cassette containing a cDNA for exons 76 through 89 and a floxed neo gene. The cDNA contains mutations in exon 76 that result in amino acid substitutions of RHRR with AHAA and disrupt the furine cleavage site. (J:144146)