Although the cellular interior is crowded with various biological 
macromolecules, the distribution of these macromolecules is highly 
inhomogeneous. Eukaryotic cells contain numerous proteinaceous membrane-less 
organelles (PMLOs), which are condensed liquid droplets formed as a result of 
the reversible and highly controlled liquid-liquid phase transitions. The 
interior of these cellular bodies represents an overcrowded milieu, since their 
protein concentrations are noticeably higher than those of the crowded cytoplasm 
and nucleoplasm. PMLOs are different in size, shape, and composition, and almost 
invariantly contain intrinsically disordered proteins (e.g., eIF4B and TDP43 in 
stress granules, TTP in P-bodies, RDE-12 in nuage, RNG105 in RNA granules, 
centrins in centrosomes, NOPP140 in nucleoli, SRSF4 in nuclear speckles, Saf-B 
in nuclear stress bodies, NOLC1 in Cajal bodies, CBP in PML nuclear bodies, SOX9 
in paraspeckles, KSRP in perinucleolar compartment, and hnRNPG and Sam68 in 
Sam68 nuclear body, to name a few), which indicates that the formation of these 
phase-separated droplets is crucially dependent on intrinsic disorder. The goal 
of this review is to show the roles of intrinsic disorder in the magic behind 
biological liquid-liquid phase transitions that lead to the formation of PMLOs.