The mutations appear to occur sporadically and spontaneously.
变异似是偶发和自发的。
Yanshanian strongly peraluminous intrusive rocks occur sporadically in the Hebei-Shanxi-Liaoning area.
燕山期强过铝质侵入岩零星出露于冀晋辽地区。
Paragangliomas of head and neck are rare tumors that occur sporadically with the prevalence of about1 in30,000 of head and neck tumors.
头颈部副神经节瘤是一罕见偶发的疾病，盛行率约占所有头颈部肿瘤的三万分之一。
CONCLUSIONS: Review of the literature confirms that cranial nerve hemangioblastomas are very rare lesions that occur sporadically or in association with VHL disease.
结论∶文献回顾证实脑神经成血管细胞瘤是极为罕见的病变，散发或与VHL病并发。
Reports of human cases occur only very sporadically, apart from Egypt and Indonesia, following the progressive control of H5N1 in poultry.
随着禽类H5N1型病毒被逐步控制，除埃及和印度尼西亚之外，仅有零星的人类患病报告。
Even worse, these conditions are likely to occur only rarely and sporadically (making the problem hard to detect and reproduce.)
更糟的是，这些情况可能很少发生且具有偶然性(使得问题很难被监测和重现)。
Even worse, these conditions are likely to occur only rarely and sporadically (making the problem hard to detect and reproduce.)
更糟的是，这些情况可能很少发生且具有偶然性(使得问题很难被监测和重现)。
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