B16F10 cells in blood. B16F10 cells labeled with CFSE were injected into mouse tail veins. After 48-72 hours, blood samples were collected directly from the heart and the percentage of CFSE-labeled cells was determined by flow cytometry. Note that blood from a control mouse (no B16F10 injection) contained essentially no CFSE-labeled cells. SSC indicates side scatter; WT, wild-type; KO, knockout.