The granules of NES-YK-sfGFP exhibited liquid-like properties. From time-lapse imaging, we observed that two independent droplets fused into one large spherical structure (Fig. ). In addition, rapid fluorescence recovery was clearly observed in FRAP studies (Fig. ). Notably, the fluidity attenuated in a length-dependent manner, where the t1/2 were 4.5 ± 1.8, 7.5 ± 1.9, and 8.9 ± 2.5 s, and the mobile fractions were 74.4 ± 13.1%, 47.3 ± 16.9%, and 26.0 ± 15.0% for YK11, YK13, and YK15 fusion, respectively (Fig. ). YK-sfGFPs without NES formed droplets with high roundness in the nucleus (Supplementary Fig. ). In the same manner as for NES-YK-sfGFPs, the fluidity in droplets gradually decreased with increasing peptide chain length (Supplementary Fig. ). The peptide length dependency proves that the YK peptide association directly affects the dynamics of droplets. This droplet formation is a YK sequence-specific event. NES-FK13/VK13-sfGFP, in which Phe or Val was used instead of Tyr, exhibited irregularly shaped aggregates, while an Ala substituent (NES-AK13-sfGFP) dispersed uniformly (Supplementary Fig. ). This trend is consistent with our previous studies 21 using XEXK repeats (X: Ile, Phe, Leu, Val, Tyr, or Trp). In a series of Lys substituents, Glu or Gln substituents (NES-YE13or NES-YQ13-sfGFPs) do not show any fluorescent puncta and an Arg substituent (NES-YR13-sfGFP) formed less-circular aggregates. Notably, fluorescent puncta were barely observed in cells for the sfGFPs fused with YK13(split) with a GGS linker at the center of YK13 peptide, YK13(Y7P) with a Tyr-to-Pro point substitution destabilizing the β-sheet secondary structure, scrambled scYK13, and YK13(K6Y/Y7K) in which only one pair of Tyr and Lys positions was exchanged (Fig. ). Furthermore, fluorescence anisotropy was not reduced, suggesting that these are monomerically dispersed (Supplementary Fig. ). Simple cationic K6 peptide also does not induce assembly. These indicate that the droplet formation is a phenomenon specific to YK repeats.