SR proteins are a highly conserved family of arginine/serine rich, spliceosome associated phosphoproteins essential for metazoan pre-mRNA splicing. SR proteins appear to act early in splicing by promoting splice site recognition and spliceosome assembly. SR proteins also play a regulatory role, because they can determine alternative splice site usage in vivo and in vitro-a particularly interesting example is the regulation of adenovirus alternative RNA splicing by dephosphorylation of SR proteins. SR proteins appear to be recruited from nuclear “speckles”, in which they are concentrated, to sites of transcription in order to spatially coordinate transcription and pre-mRNA splicing within the cell nucleus.