The RNA gel mobility shift assay was carried out as previously described (36) . The RNA oligonucleotides were 5 0 -end labeled with g-32 P-ATP (Perkin Elmer) using KinaseMax kit from Applied Biosystems following the manufacturer's instruction. Unincorporated nucleotides were removed by using G-25 Sephadex Columns. Fifteen femtomoles of radiolabeled RNA oligonucleotides were mixed with 4 pmol of SF2/ASF recombinant protein in a 20-ml binding reaction containing 2 mg yeast tRNA (Applied Biosystems). For competition, 100Â cold CD200 exon 2 oligonucleotide was added to the reaction containing the radiolabeled CD200 exon 2 oligonucleotide and SF2/ASF. After incubation for 20 min on ice, the RNA-protein complexes were separated from free RNA by electrophoresis on a 5% native polyacrylamide gel, run at 170 V for 2 h in 0.5% TBE buffer. The gel was then dried and autoradiographed at À80 C with intensifying screen.