What is chronic myeloid leukemia (CML)?

Chronic myeloid leukemia (CML) is a cancer of the blood-forming cells in the bone marrow. Bone marrow is the soft, spongy tissue inside your bones where blood cells are made. In CML, the marrow makes far too many white blood cells called myeloid cells (also called granulocytes). CML is caused by a gene change that happens during a person's life. It is not passed down in families. Two chromosomes, 9 and 22, swap pieces. The shortened chromosome 22 is called the Philadelphia chromosome. This swap joins two genes, BCR and ABL1, into one new gene called BCR-ABL. That gene makes a protein that acts like a stuck "on" switch, telling the cells to keep growing.

When should I call my care team?

Fever of 100.4 F (38 C) or higher, or shaking chills; New or worse shortness of breath, cough, or chest pain (possible fluid around the lungs, especially on dasatinib); Chest pain or pressure, sudden weakness or numbness on one side, trouble speaking, or sudden vision changes (possible heart attack or stroke); Swelling, pain, or warmth in one leg, or sudden trouble breathing (possible blood clot); Bleeding that will not stop, black or bloody stools, or many new bruises or tiny red spots; Severe belly pain, especially in the upper belly or spreading to the back; Fast weight gain, swelling that gets worse quickly, or puffiness that makes it hard to see; Yellow skin or eyes, or dark urine; A racing or irregular heartbeat, fainting, or feeling very dizzy; Vomiting or diarrhea that will not stop, or you cannot keep down your TKI pills.

Chronic myeloid leukemia (CML)

CML is a blood cancer that daily targeted pills can control very well, so most people today live close to a normal life span, and some can even stop treatment.

Hi, I'm Rosa. Today we'll go through chronic myeloid leukemia, or CML, together, one step at a time. I'll keep it simple, and you can pause me, go back, or skip ahead whenever you like. Just so you know, this is for learning only. It is not medical advice, so please check everything with your own doctor and care team. Okay, let's get started.

What CML is

Let's start with the basics. CML stands for chronic myeloid leukemia. It is a cancer of blood-forming cells in the bone marrow. The marrow makes far too many white blood cells. CML is caused by a gene change that happens during life. It is not passed down in families. Two chromosomes swap pieces. This makes a new gene called BCR-ABL. It acts like a switch stuck on. Here is the good news. Daily pills can block that switch. Most people today live close to a normal life span.

The phases of CML

Let's talk briefly about the phases. CML has phases instead of types. Most people are found in the chronic phase. That is the early phase, and pills work very well there. In the accelerated phase, CML is getting harder to control. In the blast phase, CML acts like a fast-growing leukemia. It needs strong treatment right away. With daily pills, very few people move past the chronic phase. Your care team will tell you which phase you have.

Tests you may have

Now let's talk about tests. A blood count often shows a very high white cell count. A bone marrow test is done at the start. A needle takes a small sample from the hip bone. You get numbing medicine first. It confirms the phase and looks at your chromosomes. After that, the most important test is a blood test called PCR. It measures how much of the CML gene is left. You will have it every few months, for as long as you have CML.

Risk and response goals

Next up, risk and goals. CML is not staged by tumor size. Doctors give a risk score at the start. It uses your age, spleen size, and blood counts. Then your team watches your PCR number drop. They compare it to goals at three, six, and twelve months. Meeting those goals means treatment is working well. Missing one does not mean failure. It means your team will look for the reason, and may switch your pill. Over time, the goal is a very low number that stays low.

Gene tests

Before we move to treatment, there are some things to know. Everyone with CML has the BCR-ABL gene. That is why the pills work so well. They are aimed right at it. Your PCR results are given as a percent. Lower is better. If your number goes up, a test can look for small changes in the gene. Some changes mean a different pill will work better. Ask for your results after each test. Many people keep a log. The Questions to ask page can help you prepare.

Main treatments

Now let's talk about treatment. Almost everyone with CML takes a daily pill called a TKI. There are several choices, like imatinib, dasatinib, nilotinib, bosutinib, and asciminib. They all block the CML gene. They differ in side effects and how you take them. Taking your pill every day is the most important thing you can do. If one pill does not suit you, another often will. Some people who do very well for years can try stopping, with close blood tests. Clinical trials may offer new options too.

Side effects and help

Let's look at side effects. Most are mild, and many get better after the first few months. Some people feel tired, or get muscle cramps. Some have puffy eyes, swelling, nausea, or diarrhea. One pill can cause fluid around the lungs, so report new trouble breathing. Some pills can affect the heart, blood sugar, or cholesterol. Tell your team about any side effect, even a small one. A dose change or a new pill can often help. Always check new medicines with your pharmacist. We have a whole side effects guide on this site.

Living well with CML

Now let's talk about living well. For most people, CML is a long-term condition. A daily pill and regular blood tests keep it in check. Many people work, travel, and raise families. A pill box or phone alarm can help you remember your dose. If cost or side effects make it hard, tell your team. Help is often available. Keep your blood tests, even when you feel fine. If you are planning a pregnancy, talk with your team first. Call for a fever, chest pain, or trouble breathing. If it feels very bad, call 911. Also visit our Screening, prevention and survivorship section.

You did great today. Thank you for learning with me. Write down your questions and bring them to your next visit. You can call your care team any time, day or night. One pill a day, and regular blood tests, can carry you a long way. You are not alone.

Call your care team right away if you have

Fever of 100.4 F (38 C) or higher, or shaking chills
New or worse shortness of breath, cough, or chest pain (possible fluid around the lungs, especially on dasatinib)
Chest pain or pressure, sudden weakness or numbness on one side, trouble speaking, or sudden vision changes (possible heart attack or stroke)
Swelling, pain, or warmth in one leg, or sudden trouble breathing (possible blood clot)
Bleeding that will not stop, black or bloody stools, or many new bruises or tiny red spots
Severe belly pain, especially in the upper belly or spreading to the back
Fast weight gain, swelling that gets worse quickly, or puffiness that makes it hard to see
Yellow skin or eyes, or dark urine
A racing or irregular heartbeat, fainting, or feeling very dizzy
Vomiting or diarrhea that will not stop, or you cannot keep down your TKI pills

Side effects: what helps

Sources

Quick answers

What is chronic myeloid leukemia (CML)?: Chronic myeloid leukemia (CML) is a cancer of the blood-forming cells in the bone marrow. Bone marrow is the soft, spongy tissue inside your bones where blood cells are made. In CML, the marrow makes far too many white blood cells called myeloid cells (also called granulocytes). CML is caused by a gene change that happens during a person's life. It is not passed down in families. Two chromosomes, 9 and 22, swap pieces. The shortened chromosome 22 is called the Philadelphia chromosome. This swap joins two genes, BCR and ABL1, into one new gene called BCR-ABL. That gene makes a protein that acts like a stuck "on" switch, telling the cells to keep growing.
When should I call my care team?: Fever of 100.4 F (38 C) or higher, or shaking chills; New or worse shortness of breath, cough, or chest pain (possible fluid around the lungs, especially on dasatinib); Chest pain or pressure, sudden weakness or numbness on one side, trouble speaking, or sudden vision changes (possible heart attack or stroke); Swelling, pain, or warmth in one leg, or sudden trouble breathing (possible blood clot); Bleeding that will not stop, black or bloody stools, or many new bruises or tiny red spots; Severe belly pain, especially in the upper belly or spreading to the back; Fast weight gain, swelling that gets worse quickly, or puffiness that makes it hard to see; Yellow skin or eyes, or dark urine; A racing or irregular heartbeat, fainting, or feeling very dizzy; Vomiting or diarrhea that will not stop, or you cannot keep down your TKI pills.

Last updated: 2026-10-27. Medically reviewed by Veli Bakalov, MD, medical oncologist (October 2026). General education from Step by Step, the patient portal of HemeOncBuddy.