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。
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