RNA-binding protein (PubMed:27418600, PubMed:28072389). Specifically binds to 5'-GGGGCC-3' sequence repeats in RNA (PubMed:28072389). Essential for maintenance of peripheral motor neuron and skeletal muscle function (PubMed:26604141, PubMed:27418600, PubMed:28072389). Required for normal expression and/or alternative splicing of a number of genes in spinal cord and skeletal muscle, including the neurite outgrowth inhibitor RTN4 (PubMed:26604141, PubMed:27418600). Also contributes to normal mitochondrial respiratory function in motor neurons, via an unknown mechanism (PubMed:26604141). Interacts with KNOP1 (PubMed:15833274). Interacts with TARDBP and NUP107 (PubMed:28072389). Interacts (via N-terminus) with RBM39 (PubMed:27418600). Interacts with the SH3 domains of FYN and GRB2 (PubMed:9507006). Colocalizes with RBM39 in nuclear speckles. Inhibition of RNA synthesis, or overexpression of KNOP1, induces translocation from nuclear speckles to the nucleolus. Widely expressed, with strongest expression in skeletal muscle, heart and brain (at protein level) (PubMed:9507006, PubMed:26604141, PubMed:27418600, PubMed:28072389). Detected in spinal cord motor neurons (PubMed:28072389). During embryonic stages 12.5 dpc and 15.5 dpc, highly expressed in skeletal muscle and cardiac tissue. Down-regulated in response to insulin. Phosphorylated by FYN in vitro. May be due to an intron retention. May be due to an intron retention.