These results support the hypothesis that the splicing regulatory protein SF2/ASF, acting through binding to the ESE in exon 2 of CD200, plays an important role in controlling alternative splicing of CD200, and regulates Figure 4 . ESE deletion or overexpression of SF2/ASF affects exogenous expression patterns of CD200 isoforms. Ten micrograms of the wild-type minigene construct, the minigene construct with the ESE site deleted, the minigene construct plus SF2/ASF expression vector, or the minigene construct with the ESE site deleted plus SF2/ASF expression vector was transfected into Daudi or SK-N cells by electroporation. After 48 h, cells were collected and total RNA was isolated for real-time RT-PCR.The expression levels of the full-length and truncated CD200 as well as total CD200 in Daudi (A) and SK-N (B) were normalized to the housekeeping genes GAPDH and HPRT. The data shown are expression levels of full-length or truncated CD200 relative to total CD200. The data represent the mean ± SE (three independent experiments, triplicate determinations). the relative ratio of expression of full length to truncated CD200.