Identification of a putative ESE for SF2/ASF binding does not provide direct evidence that SF2/ASF recognizes and binds to the ESE. To examine whether the identified ESE in exon 2 is bound by SF2/ASF, we performed RNA-EMSA using RNA radiolabeled oligonucleotides bearing the ESE in exon 2 and a recombinant SF2/ASF with ÁRS domain to reduce nonspecific binding. The result showed a binding of SF2/ASF to the ESE and the binding is specific because either mutated ESE or 100Â cold oligonucleotides abolished the binding.