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T-cell development is a highly regulated process that occurs primarily in the thymus and is essential for adaptive immunity. T-cells originate from hematopoietic stem cells in the bone marrow, which give rise to common lymphoid progenitors. These progenitors migrate to the thymus, where they undergo a series of differentiation and selection events to become mature, functional T-cells capable of recognizing specific antigens. Within the thymus, progenitor cells first enter the thymic cortex as double-negative (DN) cells, meaning they lack both CD4 and CD8 surface markers. DN cells progress through four stages (DN1–DN4), characterized by distinct patterns of gene expression, receptor rearrangement, and proliferation. One of _____ ____ ____ ______ ____ ___ ___ ___.
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