Abstract: The present invention comprises cDNA coding for histidinol dehydrogenase from plants, the final step in histidine biosynthesis. The invention also comprises a novel method of purifying histidinol dehydrogenase from plants to essential honogeneity, the purified histidinol dehydrogenase, an assay for identifying inhibitors of histidinol dehydrogenase, an assay to identify mutants of histidinol dehydrogenase that are not inhibited by inhibitors of wild-type histidinol dehydrogenase, the inhibitors so identified as well as herbicide compositions containing them, the non-inhibited mutants of histidinol dehydrogenase, transgenic crop plants containing the non-inhibited mutants of histidinol dehydrogenase, and methods of treating weeds utilizing the application of histidinol dehydrogenase inhibitors to the transgenic crops containing the non-inhibited mutants of histidinol dehydrogenase.