Since the ESE described above is known to contain a putative binding site for SF2/ASF, we investigated whether SF2/ASF binds to the ESE. An RNA-EMSA was performed. As shown in Figure 5 , an RNA-protein complex was detected after the SF2/ASF recombinant protein with ÁRS domain was mixed with a radiolabeled RNA oligonucleotide containing the ESE site. This protein/RNA interaction is specific since SF2/ASF did not bind to a radiolabeled RNA oligonucleotide containing mutated ESE site and the above binding was eliminated by competing 100Â unlabelled oligonucleotide containing the same ESE ( Figure 5 ). Moreover, this binding was not competed by the same level of cold oligonucleotide with the ESE site mutated (data not shown). As previously described, the full-length CD200 was expressed predominantly in brain and neuronal cells. One explanation of this observation is that the expression of SF2/ASF is higher in neuronal cells and brain. To test this hypothesis, we assessed SF2/ASF levels in Daudi and SK-N cells by Western blotting. As shown in Figure 6A , the natural level of SF2/ASF was clearly higher in SK-N cells than in Daudi cells.