text: ASF/SF2 has the ability to be phosphorylated at the serines in its RS domain by the SR specific protein kinase, SRPK1. SRPK1 and ASF/SF2 form an unusually stable complex of apparent K of 50nM. SRPK1 selectively phosphorylates up to twelve serines in the RS domain of ASF/SF2 through a directional and processive mechanism, moving from the C terminus to the N terminus. This multi-phosphorylation directs ASF/SF2 to the nucleus, influencing a number of protein-protein interactions associated with splicing. ASF/SF2's function in export of mature mRNA from the nucleus is dependent on its phosphorylation state; dephosphorylation of ASF/SF2 facilitates binding to TAP, while phosphorylation directs ASF/SF2 to nuclear speckles. Both phosphorylation and dephosphorylation of ASF/SF2 are important and necessary for proper splicing to occur, as sequential phosphorylation and dephosphorylation marks the transitions between stages in the splicing process. In addition, hypophosphorylation and hyperphosphorylation of ASF/SF2 by Clk/Sty can lead to inhibition of splicing.