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Schematic representation of the origin of testicular germ cell tumors (TGCTs) in humans. Normal spermatogonial stem cells (SSCs) Adark and Apale either undergo a cycle of self-renewal or begin to differentiate and are controlled by the expression of somatic growth factors. Classical seminomas are derived from carcinoma in situ cells (CIS). Spermatocytic seminomas originate from differentiating spermatogonia.
