The existence of discrete CD200 splice variants is cell and tissue specific Human CD200 splice variants were examined in human tissues, B cells and neuronal cells. Total RNAs from different human tissues or human B cell and neuronal cell lines were used for RT-PCR using a sense primer located in exon 1 of human CD200 and an antisense primer in exon 3. As shown in Figure 1A and B, two transcripts were detected in all the human tissues, B cell lines (Daudi, Raji and TEM) and neuronal cell lines (SK-N and HCN-1A). The larger transcript was by far the dominant one seen in the brain and neuronal cell lines. Accordingly, for subsequent experiments, the B cell line Daudi and neuronal cell line SK-N were used as representatives of the two different splicing pattern of CD200. The only tissue not expressing CD200 was human skeletal muscle. The two transcripts were purified from the agarose gel and sequenced. It was confirmed that the larger one represented an exon 2 inclusion, whereas the smaller one represented an exon 2 exclusion (CD200 tr ).