← Back
Diagnosis and tests

What the Philadelphia chromosome means

The related article (“CML: symptoms, phases and diagnosis”) explains chromosome tests. This topic focuses on one specific finding, the Philadelphia chromosome: how it arises and what it means in CML and ALL.

Last updated: ~1 min read
Chromosomes swapping pieces.
Figure 1.The 9 and 22 swap — What the Philadelphia chromosome means

01Short answer

The Philadelphia (Ph) chromosome is a shortened chromosome 22 that arises when pieces are swapped between chromosomes 9 and 22. The BCR and ABL1 genes join, forming the BCR::ABL1 fusion gene. Its protein, a tyrosine kinase, acts like a “switch stuck on”, constantly telling the cell to grow. This change is only in the leukaemia cells and is not inherited. Almost all cases of CML have Ph or BCR::ABL1, and this finding confirms the diagnosis. In ALL, especially in adults, the Ph-positive form is more common. Most importantly, there are tyrosine kinase inhibitors (TKIs) that block this protein, and treatment of Ph-positive leukaemia is planned with this in mind. The BCR::ABL1 level is tracked with PCR.

02Simple explanation

9 and 22: swapped pieces.

BCR::ABL1: a “switch stuck on”.

TKIs: targeted treatment.

03Scientific explanation

The name: this chromosome was discovered in Philadelphia in 1960 and was the first consistent chromosome change found in a cancer.

Protein forms: depending on the break point, there are p210 (mainly CML) and p190 (often ALL) forms, which affects the choice of PCR test.

04What happens in the body?

The 9 and 22 swap

Two chromosomes swap pieces and a short chromosome 22 forms.

A switch stuck on

The fusion protein signals constantly.

A two-coloured switch.
Figure 2.A switch stuck on — What the Philadelphia chromosome means

CML and ALL

The same change can occur in two different leukaemias.

Two cells with the same chromosome.
Figure 3.Ph in CML and ALL — What the Philadelphia chromosome means

05How Ph arises

Four steps.

Normal — Two chromosomes.1NormalTwo chromosomes.
Break — Pieces swap.2BreakPieces swap.
Fusion — BCR::ABL1.3FusionBCR::ABL1.
Signal — Constant growth.4SignalConstant growth.

06Ph in different leukaemias

LeukaemiaMeaning
CMLAlmost always; confirms the diagnosis
ALL (adults)Relatively common; a TKI is added to treatment
ALL (children)Uncommon
AMLVery rare

07Key terms

Translocation
A swap of pieces between chromosomes.
BCR::ABL1
The fusion gene formed in Ph.
Tyrosine kinase
A signalling protein.
TKI
Tyrosine kinase inhibitor.
Ph-positive
Having Ph or BCR::ABL1.

08Key points

  • Ph is a 9 and 22 swap.
  • BCR::ABL1 forms.
  • It sends a constant growth signal.
  • It is not inherited.
  • It occurs in CML and some ALL.
  • TKIs target it.

09Frequently asked questions

Can it pass to my children?

No, the change is only in the leukaemia cells.

Is Ph-positive ALL worse?

TKIs have changed treatment of this form; your doctor will explain the outlook.

How is it monitored?

By the BCR::ABL1 level using PCR (“Where molecular tests matter most”).

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. Can it pass to my children?

  2. Is Ph-positive ALL worse?

  3. How is it monitored?

My questions

12Sources

  1. National Cancer Institute (NCI). Definition: Philadelphia chromosome
  2. MedlinePlus Genetics. BCR gene
  3. National Cancer Institute (NCI). CML treatment (PDQ), patient version
  4. National Cancer Institute (NCI). Adult ALL treatment (PDQ), patient version
  5. National Cancer Institute (NCI). The Genetics of Cancer
  6. American Cancer Society. How Is Acute Myeloid Leukemia Classified?
  7. American Cancer Society. Typing of Acute Lymphocytic Leukemia (ALL)
  8. National Cancer Institute (NCI). Biomarker Testing for Cancer Treatment
  9. National Cancer Institute (NCI). Questions to Ask Your Doctor about Your Diagnosis
  10. National Cancer Institute (NCI). Feelings and Cancer
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
Last updated: ← Back to the list

Open the interactive site →