Chronic myelomonocytic leukemia (CMML) is a blood cancer in which the bone marrow overproduces a type of white blood cell called monocytes. It affects roughly 0.4 people per 100,000 each year, making it relatively rare, and it carries a median survival of less than four years. CMML sits in an unusual category: it shares features with both slow-growing blood disorders (myelodysplastic syndromes) and overproduction disorders (myeloproliferative neoplasms), so it’s officially classified as an overlap between the two.
How CMML Develops in the Bone Marrow
CMML begins with a mutation in a blood-forming stem cell deep in the bone marrow. That single defective cell starts producing far more monocytes and other myeloid cells than the body needs, while also making some of those cells abnormally shaped or poorly functional. Over time, the mutated clone outcompetes normal blood-forming cells.
The most common early genetic change involves a gene called TET2, found in about 46% of CMML patients. TET2 normally helps regulate which genes get turned on or off during blood cell development. When it’s knocked out, stem cells develop a strong bias toward producing monocytes and related cells. A second gene, SRSF2, is mutated in about 45% of patients and amplifies that same myeloid bias. A third gene, ASXL1 (mutated in roughly 47% of cases), plays a role in how tightly DNA is packaged, and its loss further disrupts normal blood cell development. Most patients carry mutations in more than one of these genes, and the combination drives progressively abnormal blood production.
Who Gets CMML
CMML overwhelmingly affects older adults, with a median age at diagnosis around 70. Men are diagnosed roughly twice as often as women. Younger patients do exist but are uncommon, and their disease sometimes behaves differently at the molecular level.
Common Symptoms
The symptoms of CMML depend on which aspect of the disease dominates. Some patients have a version driven mainly by low blood counts (called dysplastic CMML), while others have a version driven by overproduction of white cells (proliferative CMML). These aren’t entirely separate diseases, but they tend to show up differently.
Patients with low blood counts typically experience fatigue from anemia, frequent or stubborn infections from poorly functioning immune cells, and easy bruising or bleeding from low platelets. Patients with the proliferative form more commonly develop an enlarged spleen, which can cause fullness or discomfort in the upper left abdomen, along with constitutional symptoms like fever, unintended weight loss, and drenching night sweats. In one study of younger CMML patients, 59% had an enlarged spleen at diagnosis.
Many people are diagnosed before they notice symptoms at all, when a routine blood test reveals an unexplained rise in monocytes.
How CMML Is Diagnosed
The diagnosis hinges on one key lab finding: a persistently elevated monocyte count in the blood. Specifically, the absolute monocyte count must be at or above 0.5 × 10⁹/L, and monocytes must make up at least 10% of total white blood cells. That monocytosis has to last longer than three months to rule out temporary causes like infections.
Beyond the blood count, doctors look at a bone marrow biopsy for abnormal cell shapes (dysplasia) and check that blast cells, the immature cells that signal more aggressive disease, stay below 20% in both the blood and bone marrow. Genetic testing for mutations like TET2, SRSF2, and ASXL1 helps confirm clonality, meaning the excess monocytes are coming from a single abnormal cell line rather than a reactive process.
The 2022 updates from both the World Health Organization and the International Consensus Classification revised these diagnostic criteria, particularly around borderline cases where the monocyte count is only mildly elevated. The two systems don’t fully agree, which means some patients may be classified as CMML under one system but not the other.
Risk Categories and Prognosis
Not all CMML behaves the same way. Doctors use scoring systems to estimate how aggressively a patient’s disease is likely to progress. The most refined of these is called the CPSS-Mol, which combines four factors: the genetic risk group (based on which mutations are present), the percentage of blast cells in the bone marrow, the white blood cell count, and whether the patient needs regular red blood cell transfusions. Each factor gets a point value, and the total places patients into low, intermediate-1, intermediate-2, or high risk groups.
An older scoring system from the Groupe Francophone des Myelodysplasies takes a slightly different approach, factoring in age, hemoglobin level, white blood cell count, platelet count, and whether the patient carries an ASXL1 mutation. ASXL1 mutations are consistently linked to worse outcomes.
Overall median survival is under four years, but that number masks wide variation. Low-risk patients can live considerably longer, while high-risk patients face a much shorter timeline and a greater chance of the disease transforming into acute myeloid leukemia.
Progression to Acute Leukemia
One of the most serious risks with CMML is transformation into acute myeloid leukemia (AML). This happens in roughly 20% to 30% of patients, typically within three to five years of diagnosis. When it occurs, blast cells flood the bone marrow and blood, and the disease becomes significantly harder to treat. Certain genetic profiles, particularly high-risk mutation combinations, make transformation more likely.
Treatment Options
Treatment for CMML is tailored to risk level and symptoms. For lower-risk patients who feel well, observation with regular blood monitoring is sometimes the initial approach, since treatment carries its own side effects and doesn’t always change the long-term trajectory.
When treatment is needed, the most commonly used drugs are hypomethylating agents, which work by reversing some of the abnormal gene silencing caused by the underlying mutations. These medications produce an overall response rate of about 43% to 45%, meaning roughly half of treated patients see meaningful improvement in their blood counts or symptoms. Responses are rarely complete or permanent, but they can provide months to years of disease control.
For patients with the proliferative form who have very high white cell counts or a significantly enlarged spleen, a cell-reducing medication called hydroxyurea is often used to bring counts down quickly.
Stem Cell Transplant as a Cure
Allogeneic stem cell transplant, where a patient’s bone marrow is replaced with a healthy donor’s, is the only treatment that can cure CMML. However, it’s an intensive procedure with significant risks, including relapse and graft-versus-host disease, where the donor cells attack the patient’s tissues.
A large registry study of 209 transplanted patients reported a five-year overall survival of 30%. More recent data from specialized centers using optimized approaches, including careful patient selection and specific graft preparation techniques, have reported five-year survival as high as 71%, though these results come from highly selected patients who were in remission, had good physical fitness, and had low comorbidity burden before transplant.
Because of the risks, transplant is generally reserved for younger, fitter patients with intermediate or high-risk disease. For the majority of CMML patients, who are older and may have other health conditions, transplant is not a realistic option, and management focuses on controlling symptoms and maintaining quality of life for as long as possible.