RNA-binding protein required for embryonic and postnatal development of muscle tissue. May regulate intracellular transport and local translation of certain mRNAs. Interacts with FMR1 (By similarity). Interacts with FRX2 (By similarity). Interacts with TDRD3 (By similarity). Interacts with HABP4 (By similarity). Interacts with CYFIP2 but not with CYFIP1 (PubMed:11438699). Adjacent to Z-lines in muscles. In early embryogenesis, highest expression in somites and central nervous system. Also expressed in spinal cord, surrounding mesenchymal tissue and undifferentiated gonad. In mid-embryogenesis, most prominent in gonad and muscle tissue. Also expressed in liver, retina, telencephalon and mesencephalon. In late embryogenesis, restricted to skeletal muscle and proliferative active layers of brain. After birth, highly expressed in postmeiotic spermatids. Intermediate levels are found in heart, liver and kidney with lower levels in brain and skeletal muscle. Isoform(s) containing the 27 amino acid pocket (residues 564-590) are present in adult heart and muscle. The tandem Agenet-like domains preferentially recognize trimethylated histone peptides. Death shortly after birth. Mice expressing low levels of Fxr1 show postnatal growth retardation with reduced increase in muscle mass and strength. They die within 3 weeks of birth. Belongs to the FMR1 family.