FUS, EWSR1, and TAF15, which constitute the FET family showed robust phase separation in the absence of crowding agents at a protein concentration of 5 μM (PMID:29961577). In vivo expression of the protein in human cells, truncated to residues 1-216 (low complexity N-terminal region) led to assembly into structures with remarkably spherical morphology via liquid demixing, evidenced by GFP-tagging and fluorescence and phase-contrast microscopy. The RGG repeat region was shown to mediate poly(ADP-ribose) (PAR) binding, using in vivo expression of GFP-tagged full length protein with induced mutations in the RGG region. The number of RGG repeats determined the efficiency of PAR binding, co-localization with PAR, and hence, recruitment to sites of DNA damage. While this is not strictly required for LLPS, PAR binding serves as a nucleation event, increasing local protein concentration at sites of DNA damage. Protein localization and co-localization was followed using standard widefield, confocal, high-content microscopy and time-lapse imaging. In a cellular, in vivo context, depletion of TAF15 with RNA interference, and abrogation of PAR formation both led to the lack of granule formation at sites of DNA damage, observed using time-lapse bright-field and phase-contrast live-cell microscopy. In vitro cell-free measurements recombinant TAF15 was incubated with and without sub-stoichiometric amounts of PAR, forming spontaneous aggregates, which were consistently larger in the presence of PAR, followed by transmission electron microscopy (TEM). Consistent with in vivo results, these data provide evidence for the intrinsic ability of PAR chains to nucleate aggregation of low complexity domain-containing disordered proteins (PMID:26286827).