> 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/microarray/general/microarray-general-reference_material-list/000009599.md).

# What are the ctl probes in the Infinium Methylation Array manifest?

When present, the prefix "ctl" refers to all the control probes added into the BeadChips to assess the quality of the Infinium assay. Additionally, these probes are important for estimating background noise and for the normalization process.

For further information, see [Infinium Mouse Methylation Beadchip manifest column headings](/microarray/general/microarray-general-reference_material-list/000005894.md).

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