Caenorhabditis elegans germ cell granules (P granules) are RNA and protein condensates that are associated with RNA metabolism. In C. elegans zygotes, P granules are spatially restricted to the posterior pole and, similarly to other non-membranous organelles, their formation is driven by liquid–liquid phase separation. Seydoux and colleagues revealed that MEG-3 — a protein with an intrinsically disordered region (IDR) — is the main driver of P granule assembly and that its IDR drives phase separation, which is further enhanced by its interaction with RNA. The authors propose that the RNA-binding protein MEX-5, which is enriched at the anterior pole, limits RNA availability and thereby constrains efficient MEG-3-driven phase separation at the anterior and promotes P granule formation at the posterior. This mechanism sheds new light on the spatial regulation of non-membranous organelle assembly and distribution.