One reason it's been harder to identify stem cells is that they're so rare; the other reason is that it's hard to identify the characteristics of them and it's taken many years to work this out.

干细胞难以识别的原因之一是,这些细胞很少,另一个原因就是,干细胞的特征性蛋白难以识别,我们花了好多年去解决这个问题

耶鲁公开课 - 生物医学工程探索课程节选

and it's not being taken care of because everyone's working so hard.

没有人想打扫卫生，因为大家都太忙了。

I'm afraid of 课堂 - SpeakingMax英语口语达人