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In order to increase the nutritional quality of cassava storage roots, which contain up to 85% starch of their dry weight, but are deficient in protein, a synthetic ASP1 gene encoding a storage protein rich in essential amino acids (80%) was introduced into embryogenic suspensions of cassava via Agrobacterium-mediated gene transfer. Transgenic plants were regenerated from suspension lines derived from hygromycin-resistant friable embryogenic callus lines. Molecular analysis showed the stable integration of asp1 in cassava genome and its expression at RNA level in transformed suspension lines. PCR and Southern analyses proved the transgenic nature of the regenerated plant lines. The expression of asp1 at RNA level was demonstrated by RT-PCR. The ASP1 tetramer could be detected in leaves as well as in primary roots of cultured transgenic plants by western blots. These results indicate that the nutritional improvement of cassava storage roots may be achieved by constitutive expression of asp1 in transgenic plants.