> For the complete documentation index, see [llms.txt](https://knowledge.illumina.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://knowledge.illumina.com/library-preparation/general/library-preparation-general-faq-list/000006972.md).

# How much PhiX to spike in for well balanced, base diverse libraries?

The PhiX sequencing control is not required for runs where the library pool is well-balanced in terms of base diversity, such as whole-genome and RNA-seq libraries; however, Illumina recommends spiking at least 1% PhiX into the library pool as a sequencing control and in order to obtain the error rate metric.

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| *For any feedback or questions regarding this article (Illumina Knowledge Article #6972), contact Illumina Technical Support* [*techsupport@illumina.com*](mailto:techsupport@illumina.com?subject=Question%2FFeedback%20Regarding%20Illumina%20Knowledge%20Article%20#000006972%20-%20Library%20Preparation%20\&body=Dear%20Illumina%20Technical%20Support,%0D%0A%0D%0A)*.* |


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