Seydoux explains how the PAR domains reorganize other proteins in the cytoplasm of the one-cell embryo and creates a body axis. MEX-5 is an RNA-binding protein that localizes to the anterior side of the embryo, forming a concentration gradient in the cytoplasm. Excitingly, her lab discovered that the MEX-5 gradient is caused entirely by the difference in diffusion rates between two MEX-5 species! The shift between the two species is controlled by PAR-1-mediated phosphorylation. This simple mechanism explains how a protein can become localized to a particular area of the cell without directed transport.