What Causes CMML? Did I Do Anything to Cause My CMML?

A diagnosis of chronic myelomonocytic leukemia (CMML) naturally leads to questions about how and why it happened. It is incredibly common to wonder if something you did—or didn’t do—caused your illness. The short and most important answer is no: you did not cause your CMML through your lifestyle choices, ordinary diet, or stress levels. CMML is a type of blood cancer that develops when a single blood-forming stem cell in the bone marrow acquires genetic changes (mutations) ^[1]. These mutations accumulate during a person’s lifetime, and in most cases, no single cause can be identified ^[2].

How CMML Develops in the Body

CMML is primarily driven by somatic mutations. A somatic mutation is a genetic change acquired during your life that is not present in the egg or sperm, meaning it is generally not passed down to your children ^[3][1].

As we age, it is normal for some blood stem cells to acquire mutations and begin producing a slightly larger share of our blood cells—a very common condition called clonal hematopoiesis ^[1][2]. In most people, this never causes a problem. However, in people with CMML, a single stem cell (a clone) acquires a specific combination of mutations that cause it to multiply abnormally ^[1][4]. Reported estimates vary by study population and testing method, but over 90% of people with CMML have detectable mutations in genes such as TET2, SRSF2, ASXL1, or genes in the RAS pathway ^[3][5].

These mutations change how the stem cell behaves, causing it to produce too many monocytes (a type of white blood cell) and often leading to poorly formed, dysfunctional blood cells ^[6][7]. However, because symptoms and blood counts can vary between patients, having “too many monocytes” does not necessarily mean every patient has a high total white-blood-cell count.

The mutations found on blood tests (like next-generation sequencing) usually help doctors classify your disease and estimate risk, but they do not automatically identify exactly why the disease began, nor are they automatically targets for treatment. Your hematology team will interpret them alongside your symptoms, blood counts, and other clinical findings.

Because CMML originates from genetic errors within your own cells, it is not contagious ^[1]. You cannot give it to someone else through ordinary contact, food, or household exposure, and you cannot catch it from another person.

Known and Suspected Risk Factors

While most people never learn one specific cause for their CMML, researchers have identified several risk factors associated with a higher likelihood of developing the disease:

• Age: Advancing age is the single most significant risk factor. CMML is overwhelmingly a disease of older adults, with the median age at diagnosis often reported around 75 years ^[8][9].
• Prior Cancer Treatments: Depending on the study, an estimated 9% to 11% of CMML cases are classified as therapy-related CMML. This means the disease developed years after a patient received chemotherapy or radiation therapy for an entirely different cancer ^[10][11]. The life-saving treatments used for the first cancer can damage the DNA of bone marrow cells, increasing the risk of blood cancers later on ^[12].
• Inflammation and Autoimmune Diseases: Observational studies have found that people with a history of chronic inflammatory or autoimmune disorders may have a higher risk of developing CMML ^[13][14]. However, this is an association, not proof that the inflammation directly caused the leukemia ^[13].
• Smoking: Smoking is associated with a modestly increased risk of CMML ^[13]. It is important to know that this association cannot identify the cause of an individual case. While stopping smoking remains highly worthwhile for your overall health, you should not assume personal blame for your CMML.

Having one of these risk factors does not mean you will inevitably get CMML, and having CMML does not necessarily mean it was triggered by any specific risk factor ^[13][11].

What This Means for Your Family

Because the mutations driving CMML are typically acquired during your lifetime, the disease itself is usually not passed down to your children ^[3]. However, there is an important distinction between inheriting CMML and inheriting a predisposition to blood cancers. Extremely rare inherited (germline) variants can predispose some families to myeloid neoplasms, including CMML ^[15]. A standard blood-based sequencing result alone cannot always prove whether a variant is somatic or inherited.

Routine family testing is not needed for most patients. However, you should mention it to your hematologist or ask about genetic counseling if you were diagnosed at an unusually young age, have multiple relatives with blood cancers or unexplained low blood counts, or have a personal history of multiple cancers.

If you are feeling guilty or searching your past for a reason this happened, please be gentle with yourself. There is no proven way to prevent most cases of CMML, and your focus now can safely shift from “why did this happen?” to “how do we best manage it?”