We’ve recently published a paper on membraneless organelles in Molecular Cell. A number of compartments exist in the cell that are not distinguished by membranes, including the nucleolus and Cajal bodies. We show in this work that the formation of one of these bodies is the result of a Flory-Huggin’s phase transition, and is a consequence of a very specific charge patterning of its chain. By having blocks of positive charge, housing aromatic residues, flanked by blocks and negative charge (and vice versa), phase separation (above a critical concentration, at the right salt concentration) is an inevitability. This is a neat example of important phenomena in biology having its footing firmly in polymer theory.