A loxP-flanked exon 20 with amino acid substitution of serine to alanine at position 729 (S729A), followed by a FRT-PGK-neo-FRT cassette was inserted by homologous recombination. Flp-mediated recombination removed the neo-cassette. This point mutation results in an enzyme inactive form of the protein. No change in subcellular localization was detected by immunohistochemical analysis. (J:209219)