# DNA methylation arrays and beta values (450k, EPIC)

Source: https://onco.cc/terms/methylation-arrays/  
OnCo record `methylation-arrays` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Methylation arrays measure, at hundreds of thousands of CpG sites, the fraction of DNA copies carrying a methyl mark; that fraction, between 0 and 1, is the beta value.

## Summary

The Illumina Infinium methylation assay uses BeadChip technology for genome-wide profiling of human DNA methylation, quantifying methylation at many loci (Wikipedia); the 450k array covers about 450,000 CpG sites and the EPIC array about 850,000. Each site's beta value is the methylated signal over total signal. DNA methylation in promoters typically represses transcription and is a stable record of cell identity, which is why methylation classifiers work for brain tumours and why methylation adds tissue-of-origin information to expression.

## Fields

- Kind: Term
- Last checked: 2026-09-24
- Also known as: methylation beta values; beta values; 450k array; Infinium 450k; EPIC array; MethylationEPIC; methylation array; methylation arrays; CpG methylation profile
- Tags: cansim-terms

## Notes

- Listed in the CanSim terms map 1.0.0 (docs/onco/terms.json, generated 2026-09-24), CC BY 4.0, attribution: CanSim project, an open, public-data-first cancer foundation-model programme; CanSim page path /terms/methylation-beta-values.

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Illumina_Methylation_Assay
- Wikipedia: DNA methylation: https://en.wikipedia.org/wiki/DNA_methylation
- Wikipedia: https://en.wikipedia.org/wiki/Illumina_Methylation_Assay

## Connected records

- terms: [Cancer AI vocabulary (CanSim terms map)](https://onco.cc/terms/cancer-ai-vocabulary/), [MGMT promoter methylation](https://onco.cc/terms/mgmt/), [Tissue-of-origin signal in tumour data](https://onco.cc/terms/organ-of-origin-signal/)
- technologies: [DNA methylation profiling](https://onco.cc/technologies/methylation-profiling/), [Methylation classifier for brain tumours](https://onco.cc/technologies/cns-tumour-methylation-classifier/)

---
JSON: https://onco.cc/api/v1/entities/methylation-arrays.json