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Major molecular response explained | CION Cancer Clinics
Major molecular response (MMR) means your BCR-ABL level is 0.1% or lower on the International Scale, roughly a thousandfold fall from the standard untreated level. It shows your tablet is controlling CML well, and most guidelines hope to see it by the end of the first year. It does not mean CML has gone. Here is what it means and what comes next. At CION Cancer Clinics, every leukaemia, MDS and MPN case is reviewed by our haematologist and discussed at a tumour board before a plan is agreed.
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Medically reviewed by Dr. Basudev PokhrelConsultant Haematologist · last reviewed September 2026, next review due September 2027
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The short answer
What is a major molecular response in CML?
A major molecular response (MMR) means your BCR-ABL level is 0.1% or lower on the International Scale. That is about a thousandfold fall from the standard untreated level, and it shows the tablet is keeping CML well controlled.
Why doctors care so much about it
People who reach MMR and keep it are much less likely to see their CML move to a later phase. It is the level most guidelines hope to see by the end of the first year of treatment. Once reached, the aim is to hold it, test after test.
You may see it written three ways
Reports and doctors use MMR, MR3 and a 3-log reduction to mean the same thing. Log simply counts tenfold steps. Three tenfold steps down from the starting level brings you to 0.1%.
What it is not
MMR does not mean the leukaemia has gone. A small number of CML cells remain, and they can grow back if treatment stops without a plan. It is a sign of good control, reached while you keep taking the tablet.
Who this target does not fit in the usual way
The MMR goal is set for chronic phase CML. If you were diagnosed in accelerated phase or blast crisis, or you carry a rare transcript type that the standard test cannot measure well, your team may judge response differently. The same applies if you are on a second or third tablet after earlier ones did not work. Ask which goal applies to you.
Laboratories differ in method. MMR can only be judged on a result reported on the International Scale (IS).The ladder of response
Where does MMR sit among the other responses?
Response in CML is measured at three depths, each more sensitive than the last.
Haematological response
Your blood counts return to normal and a swollen spleen shrinks. This usually comes first, within the early weeks of treatment. It says little about how much CML is left, because the blood count cannot see small numbers of leukaemia cells.
Cytogenetic response
A chromosome study of the marrow no longer finds the Philadelphia chromosome. A complete response here roughly matches a BCR-ABL level of 1% IS or lower.
Major molecular response
BCR-ABL at 0.1% IS or lower. Measured on a blood sample, so no marrow test is needed. This is the main long-term goal for most people, and the level that best protects against the disease moving forward.
Also called
- MMR or MR3
- 3-log reduction
Deep molecular response
BCR-ABL at 0.01% (MR4) or 0.0032% (MR4.5) or lower, sometimes undetectable. Holding this for years is one of the conditions for considering a planned stop.
Not sure whether this applies to you?
Ask an oncologistFrom sample to answer
How is MMR measured and confirmed?
A blood sample
Taken in a special tube and sent quickly to a laboratory that reports on the International Scale. Delays can spoil the sample, so ask the collection centre when it will be sent.
The PCR test
The lab counts BCR-ABL messages against a control gene and converts the result to an IS percentage. Your treating team does not run this test itself. It is done by an accredited laboratory.
Checking the sample quality
The report should show enough control-gene copies. A weak sample cannot reliably show MMR, however low the number looks. The lab may ask for a repeat sample.
Reading the trend
Your haematologist looks for MMR on more than one test, so a single lucky result is not mistaken for steady control. Timing since you started the tablet matters as much as the number.
On your report
Which words around MMR will you see?
- MR3
- Another name for major molecular response, 0.1% IS or lower.
- Loss of MMR
- A confirmed rise above 0.1% IS after MMR was reached. It prompts closer testing and a review of the plan.
- Deep molecular response
- MR4 or deeper. A level well below MMR.
- Undetectable
- No BCR-ABL seen in that sample. The report should also state how sensitive the test was.
- Treatment-free remission
- Remission here means CML stays controlled. This is the aim of a planned stop, with close testing afterwards.
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After you reach it
What happens once you have reached MMR?
Usually, very little changes in daily life. You keep taking the same tablet every day, and blood tests continue, often spaced a little further apart once results are stable.
If the level rises again
A single rise just above 0.1% is often repeated before anyone reacts, because the test wobbles at low levels. A confirmed loss of MMR is taken seriously. Your team checks for missed doses and other medicines, and may test for gene mutations.
If side effects are wearing you down
Reaching MMR can make it easier to talk about side effects. In some people, the team may consider a different TKI or a change in the plan. Never lower the dose or stop on your own. Only your haematologist can judge whether a change is safe.
Who should not expect to stop
MMR alone is not enough to try stopping treatment. That is only discussed for people with deeper responses held for years, in chronic phase, with reliable testing close at hand.
Commonly believed
What do people often get wrong about MMR?
"MMR means the CML is finished."
It means good control, not an end. CML cells remain, and daily treatment is what keeps them in check. Testing continues for that reason.
"If we have not reached MMR by the first year, the tablet has failed."
Not necessarily. Guidelines treat a level between 0.1% and 1% at that point as a warning zone, not failure. Your team may watch more closely or look for a reason first.
"Once MMR is reached, tests are a waste of money."
A rise is easiest to act on when it is caught early. Regular tests are how you keep the control you have worked for.
"A deeper response on one test means we can relax the routine."
Deep results are encouraging, but the routine is what produced them. Keep the daily tablet and test schedule unless your haematologist changes it.
Did you know
The BCR-ABL test is so sensitive that it can pick up one leukaemia message among many thousands of normal ones. That sensitivity is why small ups and downs are expected, and why your haematologist reads the trend instead of any single result.
Questions we are asked
Common questions about major molecular response
My report says MMR achieved. What does that mean for me?
It means your BCR-ABL level is at or below 0.1% IS, and the tablet is controlling CML well. Keep taking it exactly as prescribed and keep your test schedule. Your haematologist will tell you how often to test from here and what result would need a closer look.
Is MMR the same as remission?
People sometimes use the words loosely, but MMR is a precise lab level. Remission usually describes control of the disease in general. Treatment-free remission is a separate idea: CML staying controlled after a planned stop, which needs a deeper response first.
Can I lose MMR after reaching it?
Yes, some people do. Missed doses, interacting medicines or a gene mutation can all cause a rise. A single small rise is usually repeated first. A confirmed loss leads your team to look for the reason and review the plan with you.
How often should I test once I am in MMR?
Your haematologist decides. Tests are often more frequent until MMR is steady, then spaced out. They become more frequent again if results rise, if you switch tablets, or if a planned stop is being considered.
Does reaching MMR late matter?
It is better to reach it on time, but reaching it later still counts. What matters most is that the level keeps falling and then holds. Your team judges timing together with your earlier results and how steadily you have taken the tablet.
Do all TKIs aim for the same MMR level?
Yes. The definition of MMR, 0.1% IS or lower, does not change with the tablet. Newer TKIs may help some people reach it sooner. The choice of tablet depends on your health, other conditions and side effects, and is made with your haematologist.
Can I get MMR confirmed at any laboratory?
Choose a lab that reports on the International Scale and shows the control-gene count. Staying with the same lab keeps your trend easy to read. Ask your team which laboratories they rely on near where you live.
Can CION review my results?
Yes. Send every BCR-ABL report in date order with the lab name and your current tablet. CION's haematology team will read them as a trend and explain where you stand. Call the helpline to arrange a review.
Your Haematologist
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Dr. Basudev Pokhrel reviews blood counts, transfusion needs and blood disorders, and works with the CION tumour board on blood cancers.
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Sources
- Leukemia & Lymphoma Society — Chronic Myeloid Leukemia
- National Cancer Institute — Chronic Myelogenous Leukemia Treatment (PDQ) - Patient Version
- Cancer.Net — Leukemia - Chronic Myeloid - CML
- Cancer Research UK — Chronic myeloid leukaemia (CML)
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Reading Your BCR-ABL Report and the IS ScaleHow to read the number behind MMRCML Treatment Milestones at 3, 6 and 12 MonthsWhen MMR is expected in the first yearTreatment-Free Remission: Who Can Try StoppingThe deeper response needed before a planned stopWhat Happens If You Miss a MilestoneIf MMR has not come on time
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Send us your BCR-ABL reports in date order. Our haematology team will explain where you stand and what to watch. One helpline serves every CION centre.