In vivo transfer of DiR dye from human leukemic cells toward mouse cells. (A) Sublethally irradiated mice were intravenously injected with 20 × 10⁶ HL60-eGFP–bright cells stained with 5μM DIR and preincubated overnight at 37°C before the infusion. BM cells were analyzed by FC 4 days afterward. (B left panels) Frequency and analysis of DiR⁺ cells within human (eGFP⁺) and mouse (eGFP⁻) cells. (Right panels) Detailed phenotype of live cells positive for DiR. (C) FC quantification of human cell frequencies within the BM of recipients based on the detection of DiR⁺ or eGFP⁺ events (n = 4). (D) Determination of the absolute contribution of mouse and human cells within the total DiR signal: for each animal (n = 4), the absolute contribution of human (eGFP⁺) DiR⁺ cells was calculated by multiplying the mean fluorescence intensity by the number of events. The same operation was done for mouse cells (eGFP⁻) and for the total population of DiR⁺ cells. Results are expressed as a percentage of total DiR absolute signals.