Adult liver and bone marrow SP cells and identification of the liver CD45⁺ SP tip. FACS analysis of Hoechst-stained single-cell suspensions identifies a side population (SP) of cells in murine adult liver (A), Ficolled adult liver (B), and Ficolled bone marrow (C). Closer inspection of the dot plot of liver SP shown in panel B reveals a cluster of SP cells with highly efficient dye-efflux properties (termed SP tip cells) that is distinct from cells found higher in the SP gate (SP shoulder cells; gates from B are redrawn in D to show cell frequencies of tip and shoulder subpopulations). Hoechst dye efflux in SP cells is inhibited by verapamil (E). Frequencies of cells expressing the panhematopoietic surface marker, CD45, in liver SP tip, shoulder, and non-SP cells are shown in panels F, G, and H, respectively. Rare CD45⁺ cells, representing just 3% of SP tip cells, are identified in panel F. Numbers in all graphs represent percentage of events contained in the illustrated gate.