The Philadelphia Chromosome: t(9;22)(q34;q11.2) Explained
Last updated: October 22, 2026
46,XY,t(9;22)(q34;q11.2)[20]
The Philadelphia chromosome is the shortened chromosome 22 produced by a reciprocal translocation between chromosomes 9 and 22, t(9;22)(q34;q11.2). It is the hallmark of chronic myeloid leukemia (CML). This page explains the notation, what to look for in a bone marrow karyotype, and the variants you may see.
What the notation means
A typical result from the bone marrow of a man with chronic myeloid leukemia (CML) is written 46,XY,t(9;22)(q34;q11.2)[20].
| Part | Meaning |
|---|---|
| 46 | 46 chromosomes. A balanced translocation moves material between chromosomes without changing the count. |
| XY | Male sex chromosomes |
| t | A reciprocal translocation |
| (9;22) | Between chromosomes 9 and 22 |
| (q34;q11.2) | The breakpoints: band q34 on the long arm of 9, and band q11.2 on the long arm of 22, in the same order as the chromosomes |
| [20] | The translocation was found in all 20 metaphases analyzed |
The Philadelphia chromosome is the derivative chromosome 22 from this translocation, the shortened 22 that has lost most of its long arm to chromosome 9 and gained a small piece of 9q. It was the first consistent chromosome change ever linked to a cancer.
What to look for in the karyotype
- A small chromosome 22. One 22 is clearly shorter than its homolog, because most of its long arm is missing. This is the Philadelphia chromosome.
- A longer chromosome 9. One 9 has extra material at the end of its long arm, the part that came from 22.
- The same change in every abnormal cell. Count how many metaphases carry the translocation and how many are normal, and report the numbers in square brackets.
- Additional changes. Look for extra abnormalities on top of the translocation, which can signal disease progression.
Bone marrow morphology is often poor. Leukemic metaphases are frequently short and fuzzy, and the Philadelphia chromosome can be mistaken for a 21 or a deleted 22. Compare both 22s and both 9s band by band before you call it.
The BCR::ABL1 fusion
The breakpoint on chromosome 22 falls within the BCR gene, and the breakpoint on chromosome 9 within the ABL1 gene. The translocation joins them into a BCR::ABL1 fusion gene on the Philadelphia chromosome. The fusion protein is a tyrosine kinase that is always switched on, which drives the uncontrolled growth of white blood cells. Tyrosine kinase inhibitors such as imatinib block this protein and have transformed the outlook for people with CML.
Because the karyotype is only one test, the fusion is also detected and monitored with FISH and with quantitative RT-PCR, which can measure very low levels of disease during treatment.
Where the Philadelphia chromosome occurs
- Chronic myeloid leukemia: found in about 95 percent of cases, and BCR::ABL1 is required for the diagnosis.
- Acute lymphoblastic leukemia: found in a substantial minority of adult cases and a small share of childhood cases.
- Acute myeloid leukemia: rarely.
Variant and related karyotypes
| Karyotype | Meaning |
|---|---|
| 46,XX,t(9;22)(q34;q11.2)[17]/46,XX[3] | The translocation in 17 cells, with 3 normal cells |
| 47,XY,+8,t(9;22)(q34;q11.2)[20] | An additional trisomy 8, one of the changes associated with progression |
| 47,XY,t(9;22)(q34;q11.2),+der(22)t(9;22)(q34;q11.2)[20] | An extra copy of the Philadelphia chromosome, another change associated with progression |
In a minority of cases the fusion is created by a complex or variant translocation involving a third chromosome, or is hidden from banding altogether. FISH or RT-PCR then confirms BCR::ABL1 even when the karyotype does not show the classic t(9;22). Other changes associated with progression include i(17q) and trisomy 19.
Common questions
Is the Philadelphia chromosome inherited?
No. It is an acquired change that arises in blood-forming cells, and it is not present in the rest of the body or passed to children.
Why is it written with square brackets?
Cancer karyotypes record how many metaphases carry each clone. [20] means all 20 cells analyzed had the translocation.
Keep learning
This page is for education and training. It is not medical advice and must not be used to diagnose or treat anyone. Laboratory results should be interpreted by qualified professionals following current standards.