Pap smears work by detecting abnormal cells that are cancer precursors and that can be destroyed using techniques like lasers.

巴氏涂片可以检测异常细胞，这些细胞是癌症的前兆，可以使用激光等技术来摧毁它们。

So what are today's precursors?

那么今天的预兆是什么呢？

The chemicals include deadly pesticides and their even deadlier precursors.

这些化学品包括一些能致人死地的农药成品以及危害相当的制作原料。

So what are today's precursors? I think there's just one: the ubiquitous web.

那么今天的预兆是什么呢？我认为只有一个：无所不在的互联网。

When tested in Petri dishes, the new molecules were as good as their precursors.

在培养皿实验中，修饰后的分子与其前体一样有效。