The His-SUMO-hnRNPA1 solution exhibited spontaneous temperature-dependent reversible turbidity in the absence of a crowding agent in vitro, which was revealed by differential interference contrast microscopy to reflect the presence of numerous droplets (protein localization, particle size and count). Deletion studies indicate that liquid-liquid phase separation by hnRNPA1 is mediated by the C-terminal low complexity domain (LCD). Using His-SUMO fusion constructs of truncated hnRNPA1 containing either the folded N-terminal RNA recognition motifs (A1-RRM) or the C-terminal disordered LCD (A1-LCD), the A1-LCD alone had the ability to form liquid droplets, whereas A1-RRM failed to undergo LLPS under comparable conditions to full-length hnRNPA1 (A1-FL) and all other conditions tested. Both GFP-tagged A1-LCD and a version with deletion of aa259–264 constituting the steric zipper (GFP-LCD Δhexa) efficiently incorporated into stress granules in HeLa cells in vivo. Ficoll and polyethylene glycol (PEG) were both able to promote hnRNPA1 LLPS. Lowering the NaCl concentration led to LLPS at lower A1-FL concentrations, suggesting that electrostatic interactions contributed to LLPS (PMID:26406374). After cooling down the hnRNPA1 solution from 25 °C to 4 °C negative-staining TEM showed bunches of amyloid fibrils within hnRNPA1 droplets (morphology), which was confirmed by monitoring Thioflavin T (ThT) fluorescence. As temperature was reverted back to 25 °C, the hnRNPA1 solution became clear again, and neither droplets nor amyloid fibrils was observed, indicating that the fibrils are reversible. Based on the ThT intensity, <10% of the total proteins in droplets formed reversible fibrils. However, when keeping the cloudy hnRNPA1 solution at 4 °C for several hours, the solution spontaneously became clear and the amount of reversible fibrils decreased, while a significant amount of irreversible amyloid fibrils emerged which are stable as temperature increased to 25 °C for elongated time. Segment 209-GFGGNDNFG-217, called hnRNPA1 reversible amyloid core, hnRAC1, but not the others, formed hydrogel at 4 °C. The hydrogel was composed of amyloid fibrils observed by TEM. As temperature increased to 25 °C, the fibrils disassociated spontaneously, resembling the behavior of full-length hnRNPA1. In addition to hnRAC1, another two candidates, 246-GFGNDGSNF-254 (named hnRAC2) and 260-YNDFGNY-266 (named hnRAC3) have also been identified. X-ray fibril diffraction showed that reversible hnRAC1, hnRAC2 and hnRAC3 fibrils also feature a typical cross-β architecture as seen in irreversible amyloid fibrils (PMID:31043593).