If you had been diagnosed with Chronic Myeloid Leukemia (CML) in 1980, your doctor would have been unable to offer you a cure. The expected course was a few years of stable 'chronic phase' disease, followed inevitably by an 'accelerated phase' and then a 'blast crisis' — a sudden transformation into an aggressive acute leukemia that was almost always fatal. Median survival from diagnosis was about 3-5 years.
Today, a person diagnosed with chronic phase CML can expect a life expectancy approaching that of the general population, often by taking a single pill once a day. The story of how this transformation happened is one of the most important in the history of cancer medicine — and it illustrates how fundamental biological discovery can lead, sometimes decades later, to lives saved.
What Is CML?
Chronic Myeloid Leukemia is a cancer of the bone marrow that starts when a single hematopoietic stem cell acquires a specific genetic abnormality. This abnormal cell produces an excess of white blood cells — particularly granulocytes and their precursors — which accumulate in the blood and marrow. Unlike acute leukemias, which progress rapidly, CML typically evolves slowly over months or years, giving patients time to seek treatment before becoming severely ill.
Symptoms of CML can be subtle. Many patients are diagnosed incidentally, when a routine blood test shows an elevated white blood cell count. When symptoms do occur, they typically include fatigue, unintentional weight loss, night sweats, feeling full after eating small amounts (due to enlarged spleen), and discomfort in the left upper abdomen.
The Discovery of the Philadelphia Chromosome
In 1960, two scientists in Philadelphia — Peter Nowell and David Hungerford — were examining the chromosomes of cells from CML patients under the microscope. They noticed something unusual: one of the chromosomes (chromosome 22) was shorter than normal. They called it the Philadelphia chromosome, after the city where the discovery was made. It was the first time a specific chromosomal abnormality had been consistently linked to a human cancer.
At the time, no one knew how this abnormal chromosome caused leukemia, or what to do about it. But the discovery established something profound: cancer was not just a random collection of misbehaving cells. It had specific genetic signatures. If you could find the signature, maybe one day you could target it.
It took until 1973 for Janet Rowley at the University of Chicago to work out what had actually happened. Using improved chromosome staining techniques, she showed that the Philadelphia chromosome was the result of a reciprocal translocation — a swap of material — between chromosomes 9 and 22. A piece of chromosome 9 had moved to chromosome 22, and a piece of chromosome 22 had moved to chromosome 9. It was the first translocation ever described in human cancer.
BCR-ABL1: The Fusion That Drives the Disease
Through the 1980s, molecular biologists worked out what the translocation actually did at the gene level. A gene on chromosome 22 called BCR was being fused to a gene on chromosome 9 called ABL1. The result was a new hybrid gene, BCR-ABL1, that produced a hybrid protein. This protein was a tyrosine kinase — an enzyme that normally adds phosphate groups to other proteins to regulate cell signalling — but in its fused, abnormal form, it was permanently switched on, constantly telling cells to divide, survive, and resist normal regulatory signals.
The insight was transformative. If BCR-ABL1 was causing CML, then blocking BCR-ABL1 should stop the disease. By the early 1990s, a group of researchers including Brian Druker, Nicholas Lydon, and Charles Sawyers began searching for a small molecule drug that could specifically inhibit the BCR-ABL1 kinase without harming the normal forms of the enzyme that cells needed for healthy function.
Imatinib: The First Targeted Cancer Therapy
In 1998, the first clinical trials of a compound called STI-571 — later named imatinib, and sold as Gleevec/Glivec — began. The results were astonishing. Patients with chronic phase CML who had failed prior treatments responded within weeks, often showing complete disappearance of the Philadelphia chromosome within months. Imatinib received FDA approval in May 2001, faster than almost any cancer drug before or since.
Imatinib was not the first cancer drug, but it was the first one designed specifically to target a known molecular driver of a specific cancer. Before imatinib, chemotherapy worked by damaging DNA or disrupting cell division in any rapidly dividing cell — cancer cells, yes, but also normal cells in the marrow, gut, and hair follicles. Imatinib, by contrast, was designed to fit precisely into the active site of BCR-ABL1, blocking its function while leaving most other cellular processes alone. Its side effect profile was remarkably mild compared to chemotherapy.
Today, more than 90% of patients with chronic phase CML achieve a complete hematologic response to imatinib, and most achieve major molecular responses within the first year of treatment. Long-term follow-up has shown that survival of CML patients on imatinib approaches that of matched populations without CML. For a disease that was once nearly uniformly fatal, this is nothing short of remarkable.
Second and Third Generation Inhibitors
Although imatinib is remarkably effective, some patients do not respond to it, and others develop resistance over time — usually because their CML cells acquire mutations in the BCR-ABL1 gene that prevent imatinib from binding. To address this, second-generation tyrosine kinase inhibitors (TKIs) were developed: dasatinib and nilotinib, both approved around 2006-2007. These drugs are more potent than imatinib and can overcome most resistance mutations.
Later, bosutinib and ponatinib were added to the arsenal. Ponatinib is particularly important because it is effective against the T315I mutation — a specific change in BCR-ABL1 that makes CML resistant to imatinib, dasatinib, and nilotinib. Most recently, a new class of drugs called allosteric inhibitors, exemplified by asciminib, work by binding to a different part of BCR-ABL1 and offer another option for patients with resistance.
With this expanding toolkit, most patients with CML can now be successfully managed with a TKI. The choice of which TKI to use first, how to monitor response, and when to switch is based on individual factors including age, comorbidities, disease risk, and response kinetics.
Monitoring Response: Molecular Testing
One of the most important aspects of CML care is monitoring the molecular response to treatment. This is done using a test called quantitative PCR (qPCR) for BCR-ABL1 transcripts. The test measures the amount of BCR-ABL1 mRNA in the blood relative to a control gene and reports the result on the International Scale (IS).
Response milestones are defined based on the degree of reduction: early molecular response (BCR-ABL1 ≤10% IS at 3 months), major molecular response or MMR (BCR-ABL1 ≤0.1% IS), and deep molecular response (MR4.0 or MR4.5, corresponding to 4-log or 4.5-log reduction from baseline). Patients who achieve and maintain deep molecular response for years may even be candidates for treatment-free remission — stopping their TKI under careful monitoring.
Treatment-Free Remission
One of the most exciting recent developments in CML is the observation that some patients who have been in sustained deep molecular remission on TKIs can safely stop their treatment without the disease returning. In carefully selected patients — typically those who have been in MR4.0 or deeper for at least 2 years — treatment discontinuation leads to sustained remission in roughly 40-60% of cases. Patients who do relapse can almost always re-establish remission by restarting the TKI.
This has transformed the goal of CML treatment for some patients. It is no longer just 'take this pill for the rest of your life' — it may be 'take this pill until you achieve deep remission, then consider stopping.' This concept is being studied in other cancers as well.
What About Transplant?
Before imatinib, allogeneic stem cell transplantation was the only curative option for CML. It is still occasionally used — for patients with advanced phase disease, those who fail multiple TKIs, or those with specific resistance mutations. But in the TKI era, transplant has moved from first-line therapy to a salvage option, and most patients will never need it.
What Patients Should Know
If you have been diagnosed with CML, the first thing to know is that this is one of the most treatable cancers in modern medicine. Most patients can expect near-normal life expectancy on TKI therapy. The second thing to know is that adherence matters — the evidence is clear that patients who take their TKIs reliably have better outcomes than those who skip doses or stop on their own. Side effects can usually be managed by dose adjustment or switching TKIs; stopping without medical guidance is usually the wrong choice.
Regular molecular monitoring is essential. The qPCR test provides objective information about how well the treatment is working and allows early detection of any loss of response. It is typically performed every 3 months in the first 1-2 years, then less frequently if responses are stable.
CML is the disease that taught us targeted therapy could work. From a mysterious chromosome discovered in 1960 to a fusion protein understood in the 1980s to a life-changing pill approved in 2001, the story of CML is a reminder that patient, rigorous science — sometimes unfolding over decades — can change everything. For the patients who now live full lives on a single daily tablet, the transformation is not theoretical. It is their reality.