Analysis of fluorescently labeled stem cells in vitro. lin⁻CD34⁺Sca-1⁺c-Kit⁺fetal liver stem cells were stained with the fluorescent dye PKH67.29 PKH67⁺ cells were plated onto DAS104-4 cells and cultured for 3, 4, or 5 days. Cells were obtained, gated for live cells (PI⁻) and CD45 (hematopoietic cells), and PKH67-GL fluorescence intensity was analyzed by FACS. Fluorescence intensity of stem cells in the PKH67^highregion was obtained by gating cells for high expression of PKH67 and analyzing the fluorescence intensity of these cells. Note that the fluorescence intensity of cells in the PKH67^high region goes down with time, although at a slower rate than the bulk of cells.