Blood was obtained from C57BL/6J mice, gfp homozygous mice, and C57BL/6J.gfp chimeric mice by tail venipuncture. Leukocytes were isolated by single-density gradient (Ficoll; Amersham Pharmacia Biotech, Piscataway, NJ) centrifugation, and then reacted with antibody to the endothelial cell marker CD31 (Dako, Glostrup, Denmark), followed bya rhodamine-conjugated secondary antibody (Sigma-Aldrich, St. Louis, MO). After they were washed, the cells were analyzed for two-color fluorescence (FACSCalibur; BD Biosciences). The percentage of cells both expressing gfp and reacting with anti-CD31 was analyzed from the dot plots by setting appropriate gates in the cytometry-analysis software (WinMDI FACS; Scripps Institute, La Jolla, CA).