Binds to mRNA in a sequence-independent manner. May play a role in regulation of pre-mRNA splicing or in the assembly of rRNA into ribosomal subunits. May be involved in mRNA transport. May be involved in epigenetic regulation of muscle differentiation through regulation of activity of the histone-lysine N-methyltransferase KMT5B. Homodimer and homotetramer in solution. Identified in the spliceosome C complex. Interacts with KMT5B (via C-terminus). Interacts (via N-terminus) with KPNA2 and NXF1/TAP. Interacts with F-actin with a stoichiometry of 2:1. Interacts with GARIN3, SMN1 and PABPN1 (PubMed:17103222). Localization changes during myogenesis from mainly cytoplasmic in undifferentiated myoblasts, to strongly nucleolar in early myotubes and back to cytoplasmic 5 days post-differentiation (PubMed:20970242). Localized at the Z-line in the sarcomere of matured myotubes 8 days post-differentiation (PubMed:20970242). Expressed in adult muscle, lymphocytes, fetal brain, muscle, and placenta. Also expressed in the smooth muscle of arteries and veins, the sweat glands and the epidermis. The gene represented in this entry may be involved in disease pathogenesis. Overexpression of human FRG1 in mice leads to development of facioscapulohumeral muscular dystrophy (FSHD1)-like symptoms such as kyphosis, progressive muscle dystrophy and skeletal muscle atrophy (PubMed:16341202). It also causes aberrant pre-mRNA splicing of TNNT3 and MTMR1, affects the localization and activity of KMT5B, and leads to increased levels of EID3, resulting in inhibited muscle differentiation (PubMed:23720823). These results suggest that FSHD1 results from inappropriate overexpression of FRG1 which leads to abnormal alternative splicing of specific pre-mRNAs. Belongs to the FRG1 family. Was originally thought to be located in nuclear speckles based on overexpression of the protein (PubMed:15060122). However, the endogenous protein was later shown not be expressed in nuclear speckles (PubMed:21699900).