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 was isolated from a herbicide-resistant Arabidopsis thaliana which differs from the wild type gene by only a single base pair. A "G" to "A" point mutation results in a single amino acid substitution in which the serine residue at position 653 is replaced by asparagine (S653N)

Tests using the mutant isoform of the ahas gene showed that it confers tolerance to sulfonylurea herbicides in plants. The ahas mutant isoform may, therefore, be used as a selectable marker in transgenic plants.