The codons for serines 408 and 409 in exon 9 were mutated to alanine and a loxP and FRT flanked neo cassette was inserted upstream of the 5' end of exon 9 via homologous recombination. Cre-mediated recombination removed the neo cassette. Cell surface expression of this subunit is increased to 145% of wild-type without modifying total subunit expression levels. (J:228332)