Glufosinate tolerant crops contain a transgene encoding the enzyme protein phosphinothricin N-acetyl transferase (PAT), which detoxifies glufosinate ammonium, the active component in the herbicide Liberty®.Diets containing up to 5 % PAT-Protein did not cause adverse effects in a 14-day toxicity study on rats conducted according the OECD Test Guideline 407.PAT has no structural similarity to known food allergens and is not stable at temperatures above 50 'C or at a pH outside the range 5.5 - 10. PAT is present in glufosinate tolerant crops at minute concentrations up to maximally low3 % of the total protein content. It is rapidly inactivated and degraded in gastric fluid. PAT, being a very specific enzyme, does not acetylate glutamate, the closest structural analogue to glufosinate, or any of the amino acids normally present in animals and plants.Consequently, the likelihood that PAT in glufosinate tolerant crops may involve any toxic or allergic hazards, or biochemical effects to man or animal is deemed negligible.Generally, for adequate health hazard assessment of gene modified crops, health hazards possibly linked to the gene product (PAT-Protein), need first to be examined carefully. The short term toxicity test (14 - 28 days) is best suited for intrinsic systemic types of toxicity. It allows a wide range of endpoints to be tested, using an amount of test sample that can (just) be accommodated. Secondly, various biochemical assays on the gene product are recommended as appropriate to predict how the mammalian organism handles the gene product. Depending on the outcome further testing may be indicated. Thirdly, in the case of herbicide tolerant crops, the herbicide and its plant metabolites need to be examined carefully as defined by various regulatory directives (911414 in EU). Conventional sub-chronic feeding studies with the gene modified crop are deemed insensitive and of low value for hazard assessment. They should not be part of the standard registration requirement.Development of an adequate test method for food allergenicity should be considered. In the absence of such a test method, allergenicity should be estimated on the basis of the results of the above-mentioned biochemical screening assays.