RNA splicing factor that regulates inclusion of microexons in mRNA transcripts (PubMed:35858306, PubMed:36759540). Required for motor coordination. In cortical neurons, acts redundantly with SRRM4 and regulates many alternative splicing events, including inclusion of exon 4 into the transcriptional repressor REST transcript, leading to REST inactivation. REST inactivation de-represses potassium-chloride cotransporter 2/SLC12A5 expression, resulting in the switch in GABAergic neurotransmission from immature excitatory to inhibitory (By similarity). Regulates inclusion of a microexon in TBC1D24 in the brain cortex (By similarity). In pancreatic islets, mediates microexon inclusion and regulates glucose- or amino acid-stimulated insulin secretion, as well as glucagon secretion (PubMed:36759540). Regulates microexon inclusion in the developing retina, which is required for outer segment maintenance and vision (By similarity). {ECO:0000250|UniProtKB:A0AB32TVR3, ECO:0000250|UniProtKB:Q80WV7, ECO:0000269|PubMed:35858306, ECO:0000269|PubMed:36759540}. This is the function of SRRM3 (serine/arginine repetitive matrix 3, ENSG00000177679).