Leukemia is caused by genetic mutations in blood-forming stem cells in your bone marrow. These mutations disrupt the normal instructions that tell cells when to grow, divide, and die, leading to the uncontrolled production of abnormal white blood cells. There is no single cause. Instead, leukemia develops through a combination of inherited susceptibility, environmental exposures, aging, and sometimes sheer biological bad luck.
How Normal Blood Cells Become Leukemic
Your bone marrow constantly produces new blood cells from a small pool of stem cells. These stem cells are supposed to mature into functional red blood cells, white blood cells, and platelets. In leukemia, one or more of these stem cells accumulates enough genetic damage that it begins copying itself indefinitely without maturing properly. The result is a flood of immature, dysfunctional cells that crowd out healthy blood.
This transformation doesn’t happen overnight. Cancer development is a stepwise process where increasing numbers of mutations give rise to an increasingly abnormal population of cells. Early mutations may sit quietly for years or even decades before additional genetic hits push the cell toward full-blown leukemia. Some mutations lock cells into an immature state, others switch off the self-destruct signals that would normally eliminate damaged cells, and still others accelerate division. The specific combination determines which type of leukemia develops.
The Philadelphia Chromosome and Other Genetic Drivers
One of the best-understood genetic causes is the Philadelphia chromosome, found in nearly all cases of chronic myeloid leukemia (CML). This abnormality occurs when two chromosomes accidentally swap segments, fusing two genes that don’t belong together. The resulting hybrid gene produces a protein with permanently activated growth-signaling activity. This protein pushes resting stem cells into a proliferative state, blocks normal cell death, and ramps up the cell’s energy metabolism to fuel rapid division.
Other chromosomal rearrangements drive other leukemia subtypes. Specific translocations are closely linked to particular forms of the disease, and newer classifications now recognize subtypes defined by rearrangements in genes involved in growth factor signaling. These genetic signatures aren’t just academic. They determine how aggressive the leukemia is and which treatments are most likely to work.
Aging and Pre-Leukemic Changes
Age is one of the strongest risk factors for leukemia, and the reason comes down to simple math: the longer your bone marrow has been producing cells, the more mutations accumulate. A condition called clonal hematopoiesis of indeterminate potential (CHIP) illustrates this perfectly. In CHIP, blood stem cells acquire mutations in genes associated with leukemia but haven’t yet become cancerous. At least 10% of adults over 70 carry these pre-leukemic mutations.
Most people with CHIP will never develop leukemia. The yearly risk of progression to a blood cancer is roughly 0.5 to 1%, compared to less than 0.1% for people without these mutations. But certain mutations, particularly in a tumor-suppressing gene called TP53, carry a much higher risk of transforming into acute myeloid leukemia (AML). The most commonly affected genes are those that control how DNA is read and maintained, and when they malfunction, they increase the stem cell’s ability to renew itself indefinitely, a hallmark of cancer.
Chemical and Environmental Exposures
Benzene is the environmental exposure most strongly linked to leukemia. Found in gasoline, industrial solvents, and cigarette smoke, benzene is metabolized in the body into compounds that directly damage DNA in bone marrow stem cells. These metabolites cause a specific type of oxidative damage and interfere with proteins that control how stem cells develop. What makes benzene particularly dangerous is that stem cells are biologically wired to survive rather than self-destruct when damaged, so instead of dying off, they attempt error-prone repairs of their broken DNA and pass those errors along to future generations of cells.
Formaldehyde, certain pesticides, and high-dose ionizing radiation (such as that experienced by nuclear disaster survivors) are also established risk factors, though benzene remains the most thoroughly studied occupational carcinogen for blood cancers.
Smoking and Leukemia Risk
Cigarette smoke contains benzene and dozens of other carcinogens that reach the bone marrow through the bloodstream. People who have ever smoked have a 26% increased risk of developing AML compared to those who never smoked. For current smokers, the risk jumps to 42%. There’s a clear dose-response relationship: smoking more than 20 cigarettes a day raises the risk by 76%, while smoking fewer than 20 raises it by 38%. Duration matters too. Smoking for more than 20 years is associated with a 35% increase in AML risk, while smoking for fewer than 20 years shows no statistically significant increase. Smoking is also independently linked to CHIP, meaning it can set the stage for leukemia years before the disease appears.
Previous Cancer Treatment
One of the more unsettling causes of leukemia is prior treatment for a different cancer. Certain chemotherapy drugs and radiation therapy can damage DNA in bone marrow cells in ways that later trigger what’s known as therapy-related leukemia.
Different classes of cancer drugs cause damage through different mechanisms. Some transfer chemical groups onto DNA, creating highly mutation-prone lesions and crosslinks between DNA strands. Others trap cellular machinery on DNA during the copying process, preventing broken strands from being repaired and leading to large-scale chromosomal rearrangements, deletions, and inversions. Still others get incorporated directly into new DNA during cell division, acting as molecular imposters that produce dangerous base changes. Radiation works by generating reactive oxygen molecules that oxidize DNA and by directly snapping DNA strands, causing the same kinds of large chromosomal rearrangements. Therapy-related leukemia typically appears years after the original treatment and tends to be more aggressive and harder to treat than leukemia that arises on its own.
Inherited Genetic Conditions
Certain genetic syndromes substantially raise leukemia risk from birth. The clearest example is Down syndrome (trisomy 21). Children with Down syndrome face an estimated 10- to 20-fold increased risk of developing acute leukemia compared to other children. The pattern of leukemia is also different: in the general pediatric population, lymphoid leukemias outnumber myeloid leukemias by about 5 to 1, but in children with Down syndrome, the ratio is closer to 1 to 1, meaning myeloid leukemia is far more common than expected.
Down syndrome-associated leukemia has its own biology. Many cases involve a specific mutation in a gene called GATA1 that controls blood cell development, and some newborns with Down syndrome develop a temporary pre-leukemic condition called transient abnormal myelopoiesis that resolves on its own but can later progress. Interestingly, children with Down syndrome who develop lymphoblastic leukemia tend to have worse outcomes than other children with the same diagnosis, partly because of heightened sensitivity to chemotherapy side effects.
Other inherited conditions linked to increased leukemia risk include Li-Fraumeni syndrome (caused by inherited TP53 mutations), neurofibromatosis type 1, and several rare bone marrow failure syndromes. In juvenile myelomonocytic leukemia, a rare childhood form, the specific genetic background plays a major role: cases driven by certain inherited gene variants can occasionally resolve without treatment, while others are highly aggressive.
Viral Causes
At least one virus directly causes leukemia. Human T-cell leukemia virus type 1 (HTLV-1), spread through breastfeeding, sexual contact, and blood transfusions, causes adult T-cell leukemia/lymphoma. The virus is endemic in parts of Japan, the Caribbean, and sub-Saharan Africa, with roughly 5 to 10 million people infected worldwide. Only a small percentage of carriers develop leukemia, typically after decades of infection.
HTLV-1 transforms T-cells through a sophisticated two-stage process. In the early phase, a viral protein called Tax drives infected cells to multiply by hijacking multiple growth and survival pathways simultaneously. Tax switches off cell cycle brakes, disables the cell’s main tumor-suppressing protein (p53), and causes direct genetic instability, producing abnormal chromosome numbers and DNA damage. But Tax also eventually triggers a cellular aging response that would stop growth. A second viral protein, HBZ, takes over to maintain the cancer by preventing this aging response and keeping the enzyme that protects chromosome tips (telomerase) active. Together, Tax initiates the tumor and HBZ sustains it.
When Multiple Factors Converge
In most cases of leukemia, no single cause can be identified. The disease typically results from a convergence of factors: inherited genetic vulnerability, accumulated age-related mutations, and one or more environmental triggers that tip the balance. A person might carry a pre-leukemic CHIP mutation for years, then an additional genetic hit from benzene exposure or a random copying error during cell division pushes that clone over the threshold. This multi-hit model explains why leukemia becomes more common with age, why most people exposed to known carcinogens never develop it, and why it sometimes appears in young children with no identifiable risk factors at all.