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Cross section of the apical tip of the Drosophila testis, showing the locations of germ line stem cells (GSCs), somatic stem cells (SSCs), and their niches. Hub cells (green) at the apical tip of the testis form niches for both GSCs (red) and SSCs (gray, left), which generate, respectively, spermatogonial cells (yellow) and somatic cyst cells (light gray) encapsulating differentiated spermatogonial cells. The insert on top describes major signaling pathways involved in communication between GSCs and the niche cells for controlling self-renewal and proliferation.