请教一个英语语法问题！

答：本句摘自：Pilot Study Using Molecular Profiling of Patients' Tumors to Find Potential Targets and Select Treatments for Their Refractory Cancers
没有抄错。
The arrays contain probes for 51 genes for which there is a therapeutic agent that could potentially interact with that gene.
请问这句话中的“for which”是什么成分？请高手解析下该句子的语法！谢谢

因为比较专业，就是汉语也难理解。要翻译得花时间。

不过，介词+关系代词引导限制性定语从句修饰先行词并在从句中作状语。
这个应该没错的。先行词是genes,还是probes，要理解了才能判断。

你好。总感觉句子是这样的（请你不要介意）：
The arrays contain probes for 51 genes for EACH OF which there is a therapeutic agent that could potentially interact with that gene.--------否则，这个 that gene就解释不通了。

1 for EACH OF which == for each of the 51 genes --------which == the 51 genes
2 a therapeutic agent for - - - - -

for which 引导定语从句修饰先行词genes

定语从句，which为先行词