Chronic myelomonocytic leukemia (CMML) is a rare blood cancer in which the bone marrow overproduces a type of white blood cell called monocytes. It sits in an unusual place among blood cancers, sharing features of two different disease categories: myelodysplastic syndromes, where blood cells develop abnormally, and myeloproliferative neoplasms, where the marrow makes too many of certain cells. That overlap makes CMML distinct from more familiar leukemias and gives it a clinical profile all its own.
How CMML Differs From Other Leukemias
Most people hear “leukemia” and think of acute forms that strike suddenly or chronic types like CML (chronic myeloid leukemia). CMML is neither. It’s officially classified as a myelodysplastic/myeloproliferative overlap neoplasm, meaning the bone marrow is doing two things wrong at once: producing blood cells that don’t mature properly and producing too many of them. The hallmark is a persistent, abnormal rise in monocytes, a type of white blood cell that normally helps fight infection.
Under the current international consensus classification, a CMML diagnosis requires monocytes at or above 0.5 × 10⁹/L in the blood, with monocytes making up at least 10% of the total white blood cell count. Before calling it CMML, doctors must also rule out other conditions that can raise monocyte levels, including CML (by testing for a specific gene fusion called BCR-ABL1) and rarer blood cancers driven by other molecular rearrangements.
Symptoms and Early Signs
CMML often develops slowly, and some people are diagnosed through routine blood work before they notice anything wrong. When symptoms do appear, they tend to reflect two underlying problems: not enough healthy blood cells and too many abnormal ones.
Low red blood cell counts cause fatigue, weakness, and shortness of breath. Low platelets can lead to easy bruising or bleeding. Meanwhile, the overproduction of white blood cells may cause an enlarged spleen (felt as fullness or discomfort below the left rib cage). Constitutional symptoms like unexplained weight loss, night sweats, and fevers are also common. Some people develop skin lesions or fluid buildup around the lungs or heart as the disease progresses.
What Causes CMML
CMML is driven by acquired genetic mutations in blood-forming stem cells. These aren’t inherited mutations you’re born with. They develop over a lifetime, which is one reason the disease primarily affects older adults, with a median age at diagnosis in the late 60s to early 70s.
Roughly 90% of CMML patients carry detectable mutations. The most common is in a gene called TET2, found in 50% to 60% of cases. Mutations in SRSF2 and ASXL1 each appear in 40% to 50% of patients. These genes are involved in regulating how DNA is read and how blood cells develop, so when they malfunction, the bone marrow starts producing abnormal cells that crowd out healthy ones. A fourth gene, SETBP1, is mutated in 5% to 10% of cases and tends to signal more aggressive disease.
How CMML Is Diagnosed
Diagnosis starts with a complete blood count showing elevated monocytes, but confirming CMML requires several additional steps. A bone marrow biopsy is essential. Pathologists examine the marrow for signs of dysplasia (abnormally shaped, immature cells) and count the percentage of blast cells, which are very early, undeveloped blood cells. The blast percentage determines the disease subtype and directly affects prognosis.
Flow cytometry, a technique that identifies cell types based on surface markers, plays an important role in distinguishing CMML from other blood cancers that can look similar under a microscope. A specialized approach called monocyte repartitioning analyzes the specific subtypes of monocytes present, which helps separate true CMML from reactive monocytosis (a temporary, non-cancerous rise in monocytes caused by infection or inflammation). Genetic testing rounds out the workup, both to identify driver mutations and to rule out look-alike conditions.
CMML Subtypes and What They Mean
CMML is divided into subtypes based on how many blast cells appear in the blood and bone marrow. These subtypes matter because higher blast percentages indicate more aggressive disease and a greater chance of transformation into acute myeloid leukemia (AML).
- CMML-0: Fewer than 2% blasts in the blood and fewer than 5% in the bone marrow. This is the earliest, least aggressive form.
- CMML-1: 2% to 4% blasts in the blood or 5% to 9% in the bone marrow.
- CMML-2: 5% to 19% blasts in the blood, 10% to 19% in the bone marrow, or the presence of certain abnormal cell structures. This subtype carries the highest risk of progressing to AML.
Doctors also sometimes describe CMML as “dysplastic” (lower white blood cell counts, more MDS-like features) or “proliferative” (higher white blood cell counts, more overproduction). The proliferative form tends to cause more symptoms like spleen enlargement and is often treated earlier.
Predicting Outcomes With Risk Scoring
Because CMML varies so widely from person to person, doctors use prognostic scoring systems to estimate how the disease will behave. The most refined version, called CPSS-Mol, combines clinical and genetic information into a single risk score. It considers four factors: genetic risk category (based on which mutations and chromosomal changes are present), whether a patient needs red blood cell transfusions, white blood cell count, and the percentage of blasts in the bone marrow.
The scoring system divides patients into four risk groups with strikingly different outlooks. Those in the low-risk group have a median survival that extends well beyond four years, with essentially no chance of the disease transforming to AML in that time. Intermediate-1 patients have a median survival of about 64 months, with a 3% chance of AML transformation by four years. Intermediate-2 patients see a median survival of roughly 37 months, with a 21% AML risk. The high-risk group faces a median survival of about 18 months and a 48% chance of AML transformation within four years.
Treatment Approaches
Not everyone with CMML needs treatment right away. For people with early-stage, low-risk disease and no symptoms, close monitoring with regular blood tests, sometimes called “watch and wait,” is standard. Treatment is typically started when hemoglobin drops below 10 g/dL, platelets fall below 100 × 10⁹/L, blast counts rise, white blood cells climb above 30 × 10⁹/L, or symptoms like significant spleen enlargement or constitutional symptoms develop.
Hypomethylating Agents
The most widely used treatment for CMML is a class of drugs called hypomethylating agents. These work by reversing abnormal chemical modifications on DNA that silence tumor-suppressing genes. Two drugs in this class have been approved in the United States for CMML since the mid-2000s. They’re given as injections, typically in cycles, and the goal is to improve blood counts, reduce symptoms, and slow progression rather than cure the disease. Response rates vary, but many patients see meaningful improvements in their blood counts and quality of life.
Researchers are also studying combinations of hypomethylating agents with a newer oral drug that blocks a protein cancer cells rely on to survive. Early trials are evaluating this combination specifically in higher-risk CMML.
Intensive Chemotherapy
Traditional chemotherapy regimens borrowed from acute leukemia treatment have shown limited success in CMML. Remission rates hover around 40%, remissions tend to be short-lived, and relapse occurs in roughly 90% of cases. For that reason, intensive chemotherapy is generally reserved for younger patients with good overall health whose disease is rapidly worsening or has a high blast count.
Stem Cell Transplant
Allogeneic stem cell transplant, in which a patient’s bone marrow is replaced with healthy donor marrow, is the only treatment that can potentially cure CMML. However, fewer than 1% of CMML patients undergo transplant. The procedure carries significant risks including graft-versus-host disease, infection, and organ damage, and it requires a suitable donor and a patient healthy enough to tolerate the intensive preparatory regimen. It’s most often considered for younger, higher-risk patients who have a good performance status and an available matched donor.
Living With CMML
For many people, CMML is a disease managed over months to years rather than one that demands immediate aggressive treatment. Regular blood monitoring is the baseline, with visits often occurring every few weeks to months depending on the disease stage. Fatigue is one of the most persistent quality-of-life challenges, driven by chronic anemia that may require periodic red blood cell transfusions.
Because CMML affects the immune system, infections can be more frequent and more serious. Staying current on vaccinations, recognizing early signs of infection, and avoiding unnecessary exposure during periods of low blood counts are all practical steps. Spleen enlargement, when present, can cause early fullness after eating. Smaller, more frequent meals often help manage that discomfort. The psychological weight of a rare cancer diagnosis is real, and connecting with support communities for myelodysplastic and myeloproliferative diseases can make a meaningful difference in navigating both the medical and emotional dimensions of the disease.