Chronic myeloid leukemia (CML) is caused by a specific genetic abnormality called the Philadelphia chromosome, a swapped piece of DNA between two chromosomes that creates a permanently active growth signal in white blood cells. More than 95% of all CML cases carry this abnormality, making it one of the most tightly linked genetic markers in any cancer.
The Philadelphia Chromosome
The Philadelphia chromosome forms when chromosome 9 and chromosome 22 break apart and swap segments with each other. This swap, called a reciprocal translocation, produces a shortened version of chromosome 22. That shortened chromosome is the Philadelphia chromosome.
What matters isn’t the physical shortening itself. It’s what happens at the breakpoint. When the two chromosomes exchange pieces, a gene from chromosome 9 (called ABL1) fuses with a gene on chromosome 22 (called BCR). The result is a brand-new hybrid gene, BCR-ABL1, that doesn’t exist in healthy cells. This fusion gene is the direct molecular cause of CML.
How the Fusion Gene Drives Leukemia
The BCR-ABL1 fusion gene produces an abnormal protein that acts like a growth switch stuck in the “on” position. In normal cells, growth signals turn on and off as needed. The BCR-ABL1 protein is a type of enzyme (a tyrosine kinase) that signals continuously, activating multiple growth and survival pathways inside the cell at the same time. This drives white blood cells to multiply without the usual brakes, flooding the bone marrow and bloodstream with immature and abnormal cells.
Because the problem traces to a single molecular defect, CML became one of the first cancers treated with a drug designed to block that exact protein. These medications, called tyrosine kinase inhibitors, work by physically binding to the abnormal enzyme and shutting off its constant signaling. They transformed CML from a fatal diagnosis into a manageable chronic condition for most patients. The current five-year relative survival rate for CML is about 71%, largely because of these targeted therapies.
Variant and Hidden Forms of the Abnormality
Not every case looks the same under a microscope. In a small number of patients, the classic chromosome swap between 9 and 22 isn’t visible on standard chromosome analysis. These are called masked or cryptic Philadelphia translocations. The swap still happens, and the BCR-ABL1 fusion gene still forms, but additional rearrangements disguise the typical pattern. Specialized lab techniques like FISH (fluorescence in situ hybridization) can detect the fusion gene even when standard testing misses it.
There are also variant translocations where chromosome 22 swaps material with a chromosome other than 9, or where three or more chromosomes are involved in a complex chain of exchanges. Despite these differences in how the rearrangement looks, the end result is the same: the BCR-ABL1 fusion gene is produced, and it drives the disease. This is why molecular testing for the fusion gene itself, rather than relying solely on chromosome appearance, is central to diagnosing CML.
What Causes the Chromosome Swap
In most people with CML, the translocation appears to happen spontaneously during normal cell division. It is not inherited from parents and cannot be passed to children. The abnormality develops in a single blood-forming stem cell in the bone marrow at some point during a person’s life.
The one confirmed environmental risk factor is exposure to high-dose radiation. Survivors of atomic bomb blasts and nuclear reactor accidents have elevated rates of CML. Radiation therapy used to treat other cancers has also been linked to increased risk. There is some evidence that exposure to lower levels of radiation, such as from imaging scans, particularly early in life, might slightly raise leukemia risk, though the absolute increase is small. For the vast majority of patients, no clear external cause is ever identified.
How the Abnormality Is Detected
Diagnosing CML involves confirming the presence of the Philadelphia chromosome or the BCR-ABL1 fusion gene through lab tests on blood or bone marrow samples. Three main methods are used, each with different strengths.
- Chromosome banding analysis (karyotyping) examines cells under a microscope to visually identify the shortened chromosome 22. It can also detect additional chromosome changes that may affect prognosis, but it requires cells to be actively dividing and can miss cryptic rearrangements.
- FISH testing uses fluorescent probes that bind directly to the BCR and ABL1 gene regions, lighting up when the fusion is present. It’s more sensitive than standard karyotyping and can detect the abnormality even when chromosome banding looks normal.
- PCR (polymerase chain reaction) detects the BCR-ABL1 fusion at the molecular level, picking up even tiny amounts of the abnormal gene. It’s the most sensitive method and is used primarily to monitor how well treatment is working over time.
Research comparing FISH and karyotyping found that about 17% of patients classified as having a complete response by standard chromosome analysis still had detectable abnormal cells on FISH testing. This underscores why multiple testing methods are often used together, especially when tracking treatment response.
How CML Progresses Through Phases
The Philadelphia chromosome abnormality is present from the earliest stage of CML, but the disease can worsen over time as additional genetic damage accumulates. CML is classified into three phases based on the percentage of immature white blood cells (blasts) in the blood or bone marrow.
In the chronic phase, blasts make up less than 10% of cells. Most people are diagnosed in this phase, and it responds well to treatment. The accelerated phase is defined by 10% to 19% blasts, signaling that the disease is becoming harder to control. The blast phase, sometimes called blast crisis, involves 20% or more blasts and behaves more like an aggressive acute leukemia. Each phase reflects increasing genetic instability beyond the original Philadelphia chromosome, often involving new mutations that make the cancer cells more resistant to treatment.
Starting treatment during the chronic phase, before these additional changes develop, gives the best chance of long-term disease control. This is one reason early detection of the BCR-ABL1 abnormality matters so much.