Liquid-liquid phase separation, or LLPS, is a process in which proteins and other molecules inside cells spontaneously separate into dense, droplet-like compartments, much like oil separating from vinegar in a salad dressing. These droplets lack the lipid membranes that enclose most familiar cellular structures, yet they concentrate specific molecules and carry out specialized functions. Over the past decade, LLPS has emerged as a unifying explanation for how cells organize their interiors, and when the process goes awry, it plays a role in diseases from neurodegeneration to cancer.
How Cells Build Compartments Without Walls
Most people learn about cells as collections of membrane-bound organelles: the nucleus, mitochondria, the endoplasmic reticulum. But cells also contain dozens of structures that have no surrounding membrane at all. The nucleolus, where ribosome assembly begins, is one classic example. Stress granules, which appear when a cell is under duress, are another. For a long time, how these structures formed and held together was poorly understood. LLPS provides a physical explanation: certain proteins and RNA molecules, when present at high enough concentrations and under the right conditions, demix from the surrounding fluid and coalesce into concentrated droplets.
A common misconception is that “disorder” in a protein’s structure is what drives this process. Many phase-separating proteins do contain intrinsically disordered regions, stretches of amino acids that do not fold into a fixed three-dimensional shape. But the real driver is multivalency, meaning a molecule’s ability to make many weak, reversible contacts with its neighbors simultaneously. Whether a disordered region can actually promote phase separation depends on the specific chemical properties encoded in its amino acid sequence, not simply on the fact that it is disordered.1PubMed. Intrinsically disordered protein regions and phase separation: sequence determinants of assembly or lack thereof Think of it like Velcro: a single hook-and-loop contact is trivially weak, but a strip covered in thousands of them holds fast. Proteins that phase-separate tend to have many such “sticky” patches that collectively tip the balance toward demixing.
Organizing the Genome
One of the most striking roles of LLPS is in gene regulation. Cells need to turn specific genes on or off at the right time, and the machinery that reads and activates genes does not float around randomly. Instead, transcriptional coactivators like BRD4 and MED1 cluster at regions of DNA called super-enhancers, which control genes central to a cell’s identity. These clusters behave like liquid droplets: they fuse, they deform, and they concentrate the transcription apparatus to drive robust gene expression.2PubMed Central. Coactivator condensation at super-enhancers links phase separation and gene control Experiments have shown that chemicals known to disrupt condensates also disrupt these clusters, and that the disordered regions of BRD4 and MED1 can form phase-separated droplets on their own in the lab. The emerging picture is that transcriptional condensates act as hubs, pulling together the right proteins and holding them near the right genes.3PubMed Central. CTCF-mediated chromatin looping provides a topological framework for the formation of phase-separated transcriptional condensates
Phase separation also helps silence genes. Large stretches of tightly packed DNA called heterochromatin are kept in an “off” state in part by a protein called HP1 (Heterochromatin Protein 1). HP1 binds to specific chemical marks on histone proteins and can form liquid-like droplets that sequester and compact DNA, effectively walling it off from the transcription machinery.4PubMed Central. Liquid droplet formation by HP1α suggests a role for phase separation in heterochromatin Interestingly, cells can fine-tune this process: phosphorylation of HP1’s tail promotes droplet formation, and the protein’s specific affinity for certain histone marks allows it to form stable condensates at concentrations far lower than would be needed with purified protein alone in a test tube.5PubMed Central. HP1-driven phase separation recapitulates the thermodynamics and kinetics of heterochromatin condensate formation So the cell does not just passively allow phase separation to happen; it actively shapes when and where condensates form.
Signaling and Synapses
Phase separation is not limited to the nucleus. At the synapse, where one neuron communicates with the next, a dense mat of proteins called the postsynaptic density helps relay signals. The scaffold protein PSD-95 and a signaling protein called SynGAP can undergo phase separation together, forming concentrated liquid-like droplets that resemble the postsynaptic density itself.6PubMed Central. Phase Transition in Postsynaptic Densities Underlies Formation of Synaptic Complexes and Synaptic Plasticity This condensation may underlie how synapses rapidly reorganize during learning and memory.
Immune cells use the same trick. When a T cell’s receptor recognizes something foreign, the downstream signaling molecules spontaneously separate into liquid-like clusters. These clusters are selective: they recruit the kinases that propagate the signal while excluding the phosphatases that would shut it down, creating a biochemical environment that amplifies the immune response.7PubMed Central. Phase separation of signaling molecules promotes T cell receptor signal transduction More recent work has traced this further, showing that the T cell receptor component CD3ε condenses with a kinase called Lck to form what researchers call signalosomes, and that modulating this condensation directly affects T cell activation.8PubMed Central. Self-programmed dynamics of T cell receptor condensation The idea that signal transduction can be organized by phase separation, rather than just by conventional lock-and-key binding, is one of the more surprising developments in cell biology in recent years.
Stress Granules and the Cellular Emergency Response
When cells face heat, toxins, or viral infection, they rapidly shut down most protein production and redirect their resources. Part of that response involves assembling stress granules, transient droplets packed with untranslated messenger RNAs and RNA-binding proteins. Stress granules form through LLPS-mediated assembly: translation arrest causes mRNAs to accumulate, and RNA-binding proteins condense around them into liquid-like structures.9Cell Press. LLPS: Liquid-Liquid Phase Separation in Biology and Disease – Section: Summary Once the stress passes, the granules dissolve and the stored mRNAs re-enter the translation pipeline. This reversibility is a hallmark of healthy phase separation, and it is precisely what breaks down in disease.
The nucleolus provides another vivid example of phase separation at work during normal cell operations. It is a multilayered condensate where the initial steps of ribosome production take place. Super-resolution imaging has revealed that the nucleolus sorts and processes precursor ribosomal RNAs within its distinct internal layers, an arrangement that emerges naturally from the physics of phase separation rather than from membrane boundaries.10PubMed. Birth of Nucleolar Compartments: Phase Separation-Driven Ribosomal RNA Sorting and Processing
When Droplets Harden Into Disease
If healthy condensates are liquid and reversible, the pathological versions are not. In neurodegenerative diseases like amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), RNA-binding proteins that normally participate in reversible phase separation undergo a further transition: the liquid droplets solidify into gel-like or amyloid-like aggregates that the cell cannot dissolve. Two proteins at the center of this story are FUS and TDP-43. Both normally shuttle between the nucleus and cytoplasm, and both can form liquid condensates under physiological conditions. But when mutations or other insults push these proteins toward abnormal behavior, their condensates age and harden into insoluble aggregates that are the pathological hallmark of ALS and FTD.11PubMed Central. Liquid-Liquid Phase Separation of TDP-43 and FUS in Physiology and Pathology of Neurodegenerative Diseases
The transition is not instantaneous. FUS condensates, for example, start as dynamic liquid droplets but can develop amyloid-like fibrils over time, and this solidification is promoted at the surface of the condensate itself.12PubMed Central. The liquid-to-solid transition of FUS is promoted by the condensate surface Prion-like domains within these proteins, which are enriched inside the dense phase, drive the formation of stable fibrillar structures on longer timescales.13PubMed Central. The physics of liquid-to-solid transitions in multi-domain protein condensates Disease-associated mutations in FUS can make this worse. Certain arginine mutations produce condensates that grow much larger than normal and are less dynamic, consistent with a greater tendency toward aggregation.14Molecular Cell. RNA-Dependent Liquid-Liquid Phase Transition of FUS Is Impaired by Mutation – Section: Results
Tau, the protein whose tangles define Alzheimer’s disease, follows a similar trajectory. Phosphorylated or mutation-prone tau undergoes LLPS, forming droplets that become gel-like within minutes. Over days, those droplets spontaneously generate aggregates that can seed further aggregation in cells, a self-amplifying cycle that could help explain how tau pathology spreads through the brain.15PubMed Central. Tau protein liquid-liquid phase separation can initiate tau aggregation The shared theme across all these diseases is that LLPS itself is not the problem; the problem is the failure to keep condensates in their reversible, liquid state.
Cancer and Hijacked Transcription
In cancer, phase separation does not just malfunction; it gets repurposed. Certain chromosomal rearrangements produce fusion proteins that retain the ability to phase-separate but redirect it toward the wrong genes. FET fusion oncoproteins, found in sarcomas and some leukemias, form condensates at specific genomic locations and drive aberrant transcription programs that push cells toward uncontrolled growth.16PubMed Central. Phase separation of the oncogenic fusion protein EWS::FLI1 is modulated by its DNA-binding domain The EWS::FLI1 fusion, for instance, forms condensates in living cells and promotes tumorigenesis both by activating abnormal transcription and by interfering with the normal functions of the proteins it is built from. Understanding how these oncogenic condensates form and what makes them different from normal transcriptional condensates is an active area of research with clear therapeutic implications.
Viruses Build Their Own Factories
Viruses, stripped down to their essentials, need to concentrate their molecular machinery somewhere inside the host cell to replicate efficiently. Many viruses accomplish this by co-opting LLPS to build membrane-less compartments variously called viral factories, inclusion bodies, or viroplasms. These structures concentrate the viral replication machinery and the host proteins it needs, creating a microenvironment tailored for viral genome copying.17PubMed. Liquid-liquid Phase Separation in Viral Function Rotaviruses, for example, form replication factories through phase separation of two viral proteins, NSP5 and NSP2, together with viral RNA. These factories have the liquid-like properties typical of condensates and serve as the sites where new viral genomes are produced.18PubMed Central. Liquid-liquid phase separation underpins the formation of replication factories in rotaviruses Disrupting the phase separation that sustains these factories could, in principle, be a way to shut down viral replication without targeting the host cell’s own processes.
How Cells Keep Condensates in Check
Given that liquid condensates sit just a biochemical nudge away from pathological solid aggregates, cells need tight control over when and where phase separation happens. One major control mechanism is post-translational modifications: chemical tags that cells attach to proteins after they are made. Phosphorylation, acetylation, methylation, ubiquitination, and SUMOylation can all shift a protein’s tendency to phase-separate by changing its charge, shape, or binding properties.19PubMed Central. Crosstalk between protein post-translational modifications and phase separation A single phosphorylation event can be enough to tip a protein from soluble to phase-separated, or vice versa, giving the cell a rapid and reversible switch.20PubMed Central. Phase separation is regulated by post-translational modifications and participates in the developments of human diseases
This regulatory layer also matters for disease. If a mutation disrupts a modification site, the cell loses its ability to tune a condensate’s behavior, potentially allowing it to persist too long or solidify. Researchers are now building computational models to predict how specific modifications alter phase separation behavior, though the complexity of the system, where multiple modification types interact on the same protein, makes this a formidable challenge.21PubMed Central. Deep learning model of post-translational modification regulating liquid-liquid phase separation
What Drug Discovery Looks Like in a Condensate World
If disease-relevant proteins live inside condensates, then a drug has to get into the condensate to reach its target. This is a newer consideration in pharmacology. A large-scale study measuring how roughly 1,700 small molecules partition into different types of condensates found that partitioning varied nearly a million-fold across compounds but was surprisingly correlated among different condensate types, suggesting that condensates, despite being made of different macromolecules, share similar physical properties.22PubMed Central. Small-molecule properties define partitioning into biomolecular condensates
A drug’s hydrophobicity turns out to be a key factor. Condensates enriched in nonpolar residues are more hydrophobic internally, and hydrophobic drugs preferentially accumulate in them. Researchers have found a strong positive correlation between an inhibitor’s hydrophobicity and how effectively it targets phase-separated proteins, with estrogen receptor 1 condensates serving as a test case. Both the drug’s binding affinity for its target and its hydrophobicity contribute to potency, suggesting a new design principle: optimizing a drug’s partitioning behavior alongside its binding properties.23Nature Chemical Biology. Navigating condensate micropolarity to enhance small-molecule drug targeting This is still early-stage work, but it hints that the next generation of drugs for cancer and neurodegeneration might need to be designed with condensate entry in mind.
The Reproducibility Question
The LLPS field has grown so fast that it has attracted healthy skepticism about whether every reported condensate is truly phase-separated. A common experimental approach is to overexpress a protein in cells, see that it forms round puncta, and conclude that the protein undergoes LLPS. But phase separation requires crossing a saturation concentration, and overexpression artificially raises protein levels far above what the cell normally produces. A protein that forms droplets when heavily overexpressed might never reach the threshold for phase separation at its natural concentration.24PubMed Central. Considerations and challenges in studying liquid-liquid phase separation and biomolecular condensates – Section: Designation and analysis of condensates in live cells Researchers have called for more rigorous criteria: rather than relying on overexpression alone, studies should provide additional evidence such as sensitivity to concentration changes, liquid-like fusion behavior, and rapid internal molecular exchange.
New tools are helping. OptoDroplets, an engineered system that uses blue light to trigger phase separation of specific protein domains inside living cells, allows researchers to turn condensates on and off with precise spatial and temporal control.25PubMed Central. Spatiotemporal control of intracellular phase transitions using light-activated optoDroplets – Section: Results This kind of tool lets scientists ask whether forming or dissolving a condensate causes a specific functional change, moving beyond correlational observations toward causal tests.
Phase Separation Beyond Animals
Most of the attention in the LLPS field has focused on mammalian cells, but plants rely on the same organizing principle. Plant condensates have been implicated in light sensing (photomorphogenesis), the timing of flowering, and responses to drought, heat, and pathogen attack.26PubMed Central. Liquid-liquid phase separation in plants: Advances and perspectives from model species to crops This breadth suggests that LLPS is not a quirk of complex animal cells but a deeply conserved organizational strategy across life.
Evolutionary analyses support that view. In yeast and vertebrate proteins that phase-separate, the short sticky motifs that drive multivalent interactions are among the most conserved sequence elements within otherwise rapidly changing disordered regions. The spacing between these motifs, roughly one every 70 amino acid residues in one well-studied protein, appears to be under natural selection: as the disordered region grows longer across species, the number of sticky motifs scales proportionally, maintaining a characteristic density.27Genetics. The length scale of multivalent interactions is evolutionarily conserved in fungal and vertebrate phase-separating proteins – Section: Results and discussion Evolution, in other words, is not preserving the exact sequence of these disordered regions but is preserving the physical grammar that makes phase separation work.
Engineering Condensates From Scratch
As researchers learn the rules that govern which molecules phase-separate and under what conditions, synthetic biologists are beginning to build condensates from scratch. The goal is to create artificial membraneless compartments inside cells that can concentrate enzymes to speed up metabolic reactions, sequester toxic intermediates, or serve as biosensors that respond to environmental cues.28PubMed Central. Engineering synthetic biomolecular condensates These efforts borrow the design principles cells already use, multivalent sticky motifs separated by flexible spacers, and repurpose them in novel combinations. Early applications include boosting the yield of biosynthetic pathways by channeling substrates through enzyme-loaded condensates, though translating these proof-of-concept demonstrations into industrial or therapeutic applications remains a substantial engineering challenge. The work nevertheless illustrates how understanding a fundamental biophysical process can open entirely new possibilities for manipulating living systems.