Membrane-less organelles are condensed liquid functional states of RNA and proteins, mainly arising from a liquid-liquid phase transition driven by the weak interaction of multivalent proteins. These supramolecular structures localize and regulate complex biochemical reactions in the cell, such as controlling the mRNA fate or intervening in the metabolism. We have developed a synthetic system for assembling membrane-less granules, constitutively or upon induction, capable of interacting with a specific target mRNA in the cell. This system aims to mimic naturally occurring organelles and to deliver a tool with tunable biophysical properties able to have an impact in the functioning of the cell.