The present invention relates to a method for identifying proteins involved in the phosphorylation of starch and nucleic acids which code for such proteins. The present invention further relates to plant cells and plants which exhibit an altered activity of a protein which can be identified using the method according to the invention. Plant cells and plants of this type synthesise a modified starch. The present invention therefore also relates to the starch synthesised by the plant cells and plants according to the invention as well as to methods for the manufacture of this starch and to the manufacture of starch derivatives of this modified starch.