The present invention relates to processes for preparing transformed plant cells or organisms by transforming a population of plant cells which comprises at least one marker protein having a direct or indirect toxic effect for said population, with at least one nucleic acid sequence to be inserted in combination with at least one compound, preferably a DNA construct, capable of reducing the expression, amount, activity and/or function of the marker protein, with the transformed plant cells having a growth advantage over nontransformed cells, due to the action of said compound.