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Fig. 4 Zebrafish testicular GCTs are susceptible to radiation therapy. (A–C) Three different males that developed GCTs were irradiated (24 Gy whole-body irradiation in six daily fractions of 4 Gy). The animals were allowed to recover, and follow-up photographs were taken at the indicated time. Radiation treatment led to regression of the abdominal tumor in two cases (A, B) but had no effect on the third (C). (D) Hematoxylin and eosin image of a testicular GCT from a different fish 7 days after radiation treatment. (E) TUNEL staining of a part of the same GCT shown in (C). TUNEL-positive cells undergoing apoptosis are shown in red. Counterstaining with Hoechst 33342 (blue) indicates the condensed chromatin of spermatocytes. Apoptosis is occurring primarily in the spermatogonial-like cells.