The binary vector used to produce 35S::D-LDH lines contained both CaMV35S::D-LDH and pNos::nptI, thereby allowing the comparative selection of a similar amount of T1 seeds on d-lactate or kanamycin (Fig. 4). RT-PCR analyses confirmed that the selected plants indeed harbored and expressed the transgenes (not shown). Using 10 mM d-lactate or 50 mg/l kanamycin, the transformation frequencies obtained 8 days after imbibition were 1% (total seeds plated: 1900; resistant individuals: 20) and 1.7% (total seeds plated: 1953; resistant individuals: 33), respectively. Furthermore, the difference in growth rate between transgenic and non-transgenic plants on plates containing d-lactate is sufficiently large to easily detect transformants by visual inspection (Fig. 4). This demonstrates that the selection of transformants grown on d-lactate, although resulting in a lower number of individuals compared to kanamycin, was highly stringent and as rapid as the selection on kanamycin. Moreover, when these plants were transferred to greenhouse conditions they showed no obvious phenotypic differences compared to wild-type plants (not shown), indicating that d-LDH overexpression did not affect further plant development. Finally, the effect of d-lactate and its physiological precursor MG on the growth of other species like tomato and tobacco was tested. As shown in Fig. 5, both compounds are also toxic for these species, indicating the potential use of d-lactate as a selection marker for plants of agricultural interest. Some conditional-positive selection systems are more effective in certain species and regeneration systems than others, e.g., kanamycin has lower selection efficiency in cereals than in dicots. However, very small differences in fitness that may be trivial under laboratory conditions can have significant effects on the total yield of a crop under field conditions. Thus, it remains to be tested whether overexpressing d-LDH might have any effects on crop yield.