SR proteins are a conserved family of proteins involved in RNA splicing. SR proteins are named because they contain a protein domain with long repeats of serine and arginine amino acid residues, whose standard abbreviations are ""S"" and ""R"" respectively. SR proteins are 50-300 amino acids in length and composed of two domains, the RNA recognition motif (RRM) region and the RS binding domain. SR proteins are more commonly found in the nucleus than the cytoplasm, but several SR proteins are known to shuttle between the nucleus and the cytoplasm.SR proteins were discovered in the 1990s in Drosophila and in amphibian oocytes, and later in humans. In general, metazoans appear to have SR proteins and unicellular organisms lack SR proteins.SR proteins are important in constitutive and alternative pre-mRNA splicing, mRNA export, genome stabilization, nonsense-mediated decay, and translation. SR proteins alternatively splice pre-mRNA by preferentially selecting different splice sites on the pre-mRNA strands to create multiple mRNA transcripts from one pre-mRNA transcript. Once splicing is complete the SR protein may or may not remain attached to help shuttle the mRNA strand out of the nucleus. As RNA Polymerase II is transcribing DNA into RNA, SR proteins attach to newly made pre-mRNA to prevent the pre-mRNA from binding to the coding DNA strand to increase genome stabilization. Topoisomerase I and SR proteins also interact to increase genome stabilization. SR proteins can control the concentrations of specific mRNA that is successfully translated into protein by selecting for nonsense-mediated decay codons during alternative splicing. SR proteins can alternatively splice NMD codons into its own mRNA transcript to auto-regulate the concentration of SR proteins. Through the mTOR pathway and interactions with polyribosomes, SR proteins can increase translation of mRNA.Ataxia telangiectasia, neurofibromatosis type 1, several cancers, HIV-1, and spinal muscular atrophy have all been linked to alternative splicing by SR proteins.