The study investigates the liquid-to-solid phase transition of the ALS-associated protein FUS, which is involved in neurodegenerative diseases. FUS forms liquid-like compartments in both in vivo and in vitro conditions, characterized by rapid internal rearrangement, sphericity, and fusion properties. These compartments are dynamic and turnover rapidly, suggesting they serve functional roles in cellular processes such as transcription and DNA repair. However, these liquid compartments can convert into aggregated states, particularly when exposed to stress or when patient-derived mutations are present. The conversion from liquid to solid is accelerated by ALS-associated mutations, which increase the protein's tendency to aggregate. The findings suggest that the liquid-to-solid phase transition may be a critical mechanism in the development of ALS and other age-related diseases, highlighting the importance of maintaining liquid-phase homeostasis to prevent aggregation.The study investigates the liquid-to-solid phase transition of the ALS-associated protein FUS, which is involved in neurodegenerative diseases. FUS forms liquid-like compartments in both in vivo and in vitro conditions, characterized by rapid internal rearrangement, sphericity, and fusion properties. These compartments are dynamic and turnover rapidly, suggesting they serve functional roles in cellular processes such as transcription and DNA repair. However, these liquid compartments can convert into aggregated states, particularly when exposed to stress or when patient-derived mutations are present. The conversion from liquid to solid is accelerated by ALS-associated mutations, which increase the protein's tendency to aggregate. The findings suggest that the liquid-to-solid phase transition may be a critical mechanism in the development of ALS and other age-related diseases, highlighting the importance of maintaining liquid-phase homeostasis to prevent aggregation.