Abstract: The present invention is directed to enzymes and intermediates of the non-mevalonate isoprenoid pathway downstream of 2C-methyl-D-erythritol 2,4-cyclopyrophosphate and upstream of isopentenyl pyrophosphate or dimethylallyl pyrophosphate. These are used as a basis for a screening method for inhibitors of these enzymes, and a method for identifying inhibitor-resistant variants thereof. Further disclosures refer to DNA coding for said enzymes and for inhibitor-resistant variants thereof, vectors containing said DNA, cells containing said vector, and plant seeds comprising cells containing said vector. This invention is useful for the inhibition of the biosynthesis of isoprenoids in plants, bacteria and protozoa, for confering herbicide-resistance to plants, as well as for weed control in agriculture using a crop containing a herbicide-resistant gene and an effective amount of a suitable herbicide.