Annexin V is labeled with green fluorescent FITC, and the labeled Annexin V retains a high affinity for PS and can be used as a probe to detect apoptosis by fluorescence microscopy or flow cytometry. Since PS eversion occurs in the early stage of apoptosis, Annexin V-FITC staining can identify the occurrence of apoptosis in the early stage of apoptosis. Propidium Iodide (PI) is a nucleic acid dye. PI cannot penetrate the intact cell membrane of normal cells or early apoptotic cells, but can penetrate the cell membrane of membrane-damaged cells such as late apoptotic cells or necrotic cells, and bind to their DNA, causing their nuclei to become red. It can be used to distinguish early apoptotic cells from late apoptotic cells or necrotic cells. Therefore, by co-staining Annexin V-FITC with PI, cells in different apoptotic stages can be distinguished. On the scattergram of dual-color flow cytometry, Annexin V-FITC and PI double negative are normal cells, Annexin V-FITC positive and PI negative are early apoptotic cells, and Annexin V-FITC and PI double positive cells are late apoptotic cells or necrotic cells.