AHAS is an essential enzyme for many organisms as it catalyzes the
first step in the biosynthesis of the branched-chain amino acids
valine, isoleucine, and leucine.
A common mutant form of the ahas gene which differs from the
wild type gene by two mutations W548L and S627I results in an
enzyme that has tolerance to sulfonylurea herbicides in plants. The
ahas mutant isoform may, therefore, be used as a selectable marker
in transgenic plants.