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Figure 2: Thymocyte development. T-cells are developed in the thymus. The development can be defined by cell surface expression of CD4 and CD8. During the process, the thymosytes mature from CD4-CD8- double negative (DN) cells into CD4+CD8+ double positive (DP) cells and finally into single positive (SP) CD4+ or CD8+ cells. The DN cells can be further categorized into DN1, DN2, DN3 and DN4 cells. ISP: immature single positive.