In conclusion, we have identified an alternative splicing pattern for expressed human CD200 in different cells and tissues, and compared this with the pattern observed in vivo following viral infection. Our data suggest that regulation of expression of alternative splicing transcripts may be important in controlling susceptibility to viral infection. An ESE in exon 2 of CD200 is a binding site for a splicing regulatory protein, SF2/ASF, which we have shown to control the alternative splicing pattern of CD200. A drug-mediated manipulation of alternative splicing has recently been reported which includes modulation of SF2/ASF (25) . It would be of interest to know if this drug treatment alters the expression ratio of CD200 to CD200 tr and thereby produces change in immune function.