425. It is unheard of to invent a new drug or cure without first having repeated failure.
没有一次又一次的失败，就不可能发明新药物，找到新疗法。

六级词汇讲解:
it is unheard of...意思是“未曾听说过……，……不可能”.本句中，不定式短语to invent...作句子的真正主语。without表示否定，与前面的unheard形成双重否定，表达肯定的意思。

句型拓展:
It is not uncommon for students to have friendly relationship with their teachers.
学生与老师建立友好关系并不罕见。
In some countries no major business is carried on without eye contact, requiring face-to-face conversation.
在有些国家，一切重大事务都要面对面地进行，需要当面交谈。
There is no denying the truth.
真理是不容否认的。
There is no rule that has no exception.
任何规则都有例外。