When transfecting CHO cells with a CFP fused to FMRP (FMRP-CFP) or CFP alone, overexpressed FMRP-CFP induces the formation of distinct micrometer-sized foci in the cytoplasm in vivo (protein localization, particle size and count by microscopy), whereas transfection of CFP alone results in diffuse fluorescence. Three different constructs have been used for in vitro studies: full-length FMRP, truncated FMRP lacking the low-complexity region (FMRPΔLCR), and the 188-residue C-terminal low-complexity region of FMRP containing the RGG motifs. At low in vitro protein concentrations (50μM), droplet formation was not observed for any construct (particle size and count by microscopy). Upon addition of Cy3-labeled sc1 RNA, droplets were formed with FMRP and FMRP-LCR, but not with FMRPΔLCR. FMRP-LCR alone is necessary and sufficient to drive phase separation with sc1 RNA in vitro, however, FMRP-LCR-RNA phase separation is not sequence-specific and does not require sequences capable of G-quadruplex formation. FMRP-LCR-RNA droplets are dynamic and liquid-like (FRAP, morphology). Translational repressors and miRNAs partition into, and concentrate within, FMRP-LCR-RNA droplets (co-localization). Increasing salt concentrations disfavored FMRP-LCR droplet formation in the presence of sc1 RNA. FMRP-LCR phosphorylation by its in vivo kinase partner CKII increases its phase-separation propensity (particle size and count by microscopy) possibly through increasing negative charge densities of glutamic/aspartic acid-rich clusters. FMRP-LCR methylation by PRMT1 decreases phase-separation propensity without notably altering sc1 RNA-binding affinity. Arginine perturbations, either by methylation or reduction in pi character by substitution of arginine for lysine residues, perturb important interactions that facilitate the general phase-separation behavior of FMRP-LCR, as previously found for Ddx4 (PMID:30765518).