The endogenous human CD200 primer pairs for regular PCR were also used to construct an amplicon-containing plasmid (endogenous) for a standard curve. An exogenous amplicon-containing plasmid (exogenous) for a standard curve was constructed using the primers shown in Table 1 . Samples were tested in triplicate using 4 ml of first strand cDNA in a 20 ml total volume with 1Â universal master mix (Applied Biosystems). The results were normalized to that of the housekeeping gene GAPDH and HPRT. The copy number of transcripts was determined by comparison with a calibration curve of known amounts of amplicon-containing plasmid.