To investigate further the function of SF2/ASF in exon 2 inclusion or exclusion, we performed overexpression analysis by transfection of SF2/ASF expression vector to Daudi or SK-N cells and examined the fate of exon 2. As shown in Figure 4A -C, overexpression of SF2/ ASF induced exon 2 inclusion but this function was abolished in the absence of the ESE in exon 2, indicating that SF2/ASF regulates CD200 isoforms only via the ESE.