Radiation-induced mutagenesis resulted in 2 transitions and 1 transversion at condon 318 (AGC to AGT), codon 319 (GCA to GAA) and codon 321 (ACC to AGC) that caused amino acid substitutions of glutamic acid for alanine at position 319 (A319E) and serine for threonine at position 321 (S321T). The transition at codon 318 is silent. Enzyme activity is decreased to 61% of wild-type in heterozygotes and 18% of wild-type in homozygotes.