Control reactions were performed for the specificity of the real-time PCR primers. A DNA fragment, containing either exon 1, exon 2 and exon 3 or only exon 1 and exon 3, was gel purified and subcloned into pcDNA 3.0 between NotI and XhoI sites. The CD200-bearing plasmids were then linearized by XhoI. In vitro transcription was carried out using TranscriptAid T7 High Yield Transcription Kit (Fermantas Inc.) following the manufacturer's instruction. Transcribed RNA was treated with DNaseI to remove template DNA and purified by phenol:choloroform extraction and ethanol precipitation. First strand cDNA was then synthesized and real time PCR was performed. The primer pairs used for real-time PCR are shown in Figure 3A and Table 1 .