Checkpoint genes cause cell cycle arrest when DNA is damaged or DNA replication is blocked. Although a human homolog of Chk1 (hChk1) has recently been reported to be involved in the DNA damage checkpoint through phosphorylation of Cdc25A, B, and C, it is not known at which phase(s) of the cell cycle hChk1 functions and how hChk1 causes cell cycle arrest in response to DNA damage. In the present study, we demonstrate that in normal human ®broblasts (MJ90), hChk1 is expressed speci®cally at the S to M phase of the cell cycle at both the RNA and protein levels and that it is localized to the nucleus at this time. hChk1 activity, as determined by phosphorylation of Cdc25C, is readily detected at the S to M phase of the cell cycle, and DNA damage induced by UV or ionizing radiation does not enhance the expression of hChk1 or its activity. Furthermore, hChk1 exists in an active form at the S to M phase in ®broblasts derived from patients with ataxia telangiectasia (AT) which lack the functional AT mutated (ATM) gene product, suggesting that hChk1 expression is independent of functional ATM. Taken together with the ®ndings that phosphorylation of Cdc25C on serine 216 is increased at the S to M phase, it is suggested that at this particular phase of the cell cycle, even in the absence of DNA damage, hChk1 phosphorylates Cdc25C on serine 216, which is considered to be a prerequisite for the G2/M checkpoint. Thus, hChk1 may play an important role in keeping Cdc25C prepared for responding to DNA damage by phosphorylating its serine residue at 216 during the S to M phase.

Objectives Primary mediastinal germ cell tumors (PMGCTs) are rare, which often makes them difficult to treat. Herein, we examined patients with PMGCTs who underwent multimodal treatment. Methods We examined 6 patients (median age: 25 years, range: 19–27 years) with PMGCTs who underwent multimodal treatment between April 2001 and March 2015. Three patients had seminomas, 2 patients had yolk sac tumors, and 1 patient had choriocarcinoma. The median observation period was 32.5 months (range: 8–84 months). Results Three of the 6 patients received initial operation followed by 3-4 courses of chemotherapy (bleomycin, etoposide, and cisplatin (BEP) or etoposide and cisplatin (EP)). One patient developed multiple lung metastases 17 months after surgery; received salvage chemotherapy with vinblastine, ifosfamide, and cisplatin; and achieved complete remission. The remaining 3 patients received initial BEP and EP chemotherapy. Multiple lung metastases and supraclavicular lymph node metastases were detected in 2 of these patients at the initial diagnosis. The patients underwent resections to remove residual tumor after treatment, and no viable tumor cells were found. Conclusions Reliable diagnosis and immediate multimodal treatments are necessary for patients with PMGCTs. The 6 patients treated in our hospital have never experienced recurrence after the multimodal treatment.

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