# 1p/19q codeletion

Source: https://onco.cc/biomarkers/1p19q-codeletion-readout/  
OnCo record `1p19q-codeletion-readout` (Biomarker). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Loss of one copy each of chromosome arms 1p and 19q, together with an IDH mutation, defines oligodendroglioma in the WHO 2021 classification. It predicts a slower course and a good response to chemotherapy, and separates oligodendroglioma from astrocytoma.

## Summary

The whole-arm codeletion is detected by FISH, array or sequencing-based copy-number analysis; partial deletions do not count. WHO CNS5 (2021) requires IDH mutation plus 1p/19q codeletion for the diagnosis of oligodendroglioma, IDH-mutant and 1p/19q-codeleted, grades 2 to 3. The long-term RTOG 9402 and EORTC 26951 results showed that adding PCV chemotherapy to radiotherapy roughly doubled survival in codeleted tumours. Vorasidenib's label covers grade 2 oligodendroglioma by IDH mutation, not by codeletion. As an arm-level change it has no single parent gene and sits under no target here.

## Fields

- Kind: Biomarker
- Last checked: 2026-09-23
- Also known as: 1p/19q codeletion; 1p19q codeleted; 1p/19q co-deletion; 1p19q; whole-arm 1p/19q loss; oligodendroglioma marker
- Tags: biomarker; glioma; no-approval

## Sources

- WHO CNS5 (Louis et al. 2021): https://doi.org/10.1093/neuonc/noab106

## Connected records

- biomarkers: [IDH1 R132 mutation](https://onco.cc/biomarkers/idh1-r132/), [IDH2 mutation (R140 and R172)](https://onco.cc/biomarkers/idh2-mutation/), [MGMT promoter methylation](https://onco.cc/biomarkers/mgmt-promoter-methylation/)
- cancers: [Astrocytoma, IDH-mutant (grades 2 to 4)](https://onco.cc/cancers/idh-mutant-astrocytoma/), [Glioma & glioblastoma](https://onco.cc/cancers/glioblastoma/), [Oligodendroglioma, IDH-mutant and 1p/19q-codeleted](https://onco.cc/cancers/oligodendroglioma/)
- drugs: [Temozolomide](https://onco.cc/drugs/temozolomide/), [Vorasidenib](https://onco.cc/drugs/vorasidenib/)
- terms: [1p/19q codeletion](https://onco.cc/terms/1p19q-codeletion/), [Cytogenetics and karyotype](https://onco.cc/terms/cytogenetics/), [FISH / ISH (in situ hybridisation)](https://onco.cc/terms/fish/)

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