The present invention relates to the use of DNA sequences of plant origin encoding a polypeptide with aspartate carbamyltransferase (E.C. 2.1.3.2) activity for generating an assay system for finding aspartate carbamyltransferase inhibitors. For the first time, it was shown, with the aid of antisense technology, that aspartate carbamyltransferase constitutes a herbicidal target.