在于【zàiyú】

在于 links a thing to what it lies in or turns on: the cause of a problem, the key to success, the difference between two things. The subject is usually an abstract noun – 问题, 关键, 原因, 区别, 意义 – and what follows can be a noun or a whole clause. It is a written, formal word.

不在于…而在于…

The negative frame corrects a wrong idea of where the point lies: not here, but there. It is a fixture of essays and speeches.

"Is up to"

With a decision or an outcome as the subject, 在于 means "is up to" or "depends on".

No 是 after it

在于 already does the work of "is". Adding 是 doubles the verb.

• × 问题在于是时间不够 → 问题在于时间不够
• × 原因是在于他没来 → 原因在于他没来

In conversation 问题是… and 关键是… do the same job less formally.