Figure 2. Cell-cycle analysis of side population (SP) and non-SP cells. SP (A) and non-SP (B) cells were treated with Nonidet P-40 and propidium iodide (PI), and then cell-cycle status was analyzed by FACS. The M1 region of the histogram shows cells in G₀/G₁ stage, and the M2 region shows cells in G₂/M stage. The SP cell population did not contain cells in the G₂/M stage of the cell cycle.