In vivo, in C. elegans embryo cells FIB1::GFP forms large puncta that co-localize with rRNA (PMID:26351690). In vitro, in the presence of 5 μg/ml rRNA and 150 mM NaCl, FIB1 condenses into droplets at a protein concentration of ∼600 nM. FIB1 droplet microrheology reveals that they are not simple viscous liquid droplets, but are instead viscoelastic. The C-terminal methyltransferase domain (MD) of FIB1 plays a key role in promoting viscoelastic maturation of FIB1 droplets in vitro. The R/G domain (FIB1ΔC) is sufficient to form liquid-like droplets in vitro, while the MD alone (FIB1ΔN) is unable to form droplets in vitro (mutations, particle size and count by microscopy). FIB1ΔC can phase separate into liquid-like droplets in vitro, even in the absence of RNA. By contrast, full-length FIB1 requires rRNA; however, this may be a non-specific consequence of the polyanionic nature of rRNA since heparin can also drive phase separation of full-length FIB1. In vitro FIB1 and NPM1 coexist as multiphase droplets, with the NPM1 rich phase tending to partially envelope the FIB1 rich phase (PMID:27212236). The N-terminal R/G domain of FIB1 is sufficient for droplet formation, but does not encode for a separate liquid-like DFC subcompartment; instead, the C-terminal MD of FIB1, which alone is not sufficient for droplet formation, confers immiscibility with proteins in the GC (PMID:27212236).