SR proteins are essential for pre-mRNA splicing and alternative splicing. They have a modular structure with an N-terminal RNA-binding domain and a C-terminal RS domain that functions as a protein interaction domain. SR proteins can bind to pre-mRNA and recruit splicing factors, and they also mediate interactions between splicing factors bound to 5' and 3' splice sites. Recent studies suggest that all proposed SR protein functions occur during each round of splicing, and at least some functions are performed by independent SR protein molecules. SR proteins are involved in various aspects of pre-mRNA splicing, including the recruitment of U1 snRNP and U2AF to splice sites, and their functions are regulated by phosphorylation. The RS domain of SR proteins functions as a protein interaction domain, and different RS domains can mediate different protein interactions. The phosphorylation state of SR proteins can influence their subcellular localization and function in splicing. SR proteins have both exon-dependent and exon-independent functions. Exon-dependent functions include regulated 3' and 5' splice site selection, and constitutive splicing. Exon-independent functions include cross-intron interactions and tri-snRNP incorporation. Multiple SR protein molecules participate in the removal of each intron, with different SR proteins performing different functions. The regulated exon-dependent functions of SR proteins do not require dephosphorylation, while other functions may require phosphorylation. The findings suggest that SR proteins have distinct functional roles in splicing, and that their activities are regulated by phosphorylation and other factors.SR proteins are essential for pre-mRNA splicing and alternative splicing. They have a modular structure with an N-terminal RNA-binding domain and a C-terminal RS domain that functions as a protein interaction domain. SR proteins can bind to pre-mRNA and recruit splicing factors, and they also mediate interactions between splicing factors bound to 5' and 3' splice sites. Recent studies suggest that all proposed SR protein functions occur during each round of splicing, and at least some functions are performed by independent SR protein molecules. SR proteins are involved in various aspects of pre-mRNA splicing, including the recruitment of U1 snRNP and U2AF to splice sites, and their functions are regulated by phosphorylation. The RS domain of SR proteins functions as a protein interaction domain, and different RS domains can mediate different protein interactions. The phosphorylation state of SR proteins can influence their subcellular localization and function in splicing. SR proteins have both exon-dependent and exon-independent functions. Exon-dependent functions include regulated 3' and 5' splice site selection, and constitutive splicing. Exon-independent functions include cross-intron interactions and tri-snRNP incorporation. Multiple SR protein molecules participate in the removal of each intron, with different SR proteins performing different functions. The regulated exon-dependent functions of SR proteins do not require dephosphorylation, while other functions may require phosphorylation. The findings suggest that SR proteins have distinct functional roles in splicing, and that their activities are regulated by phosphorylation and other factors.