Inclusion Criteria:

          1. CML in first chronic phase.

          2. Demonstration of BCR-ABL1 positivity at/shortly after original diagnosis.

          3. Written Informed Consent

          4. Must have received TKI treatment for at least 3 years.

          5. At least 3 molecular results over the preceding 12 months, that fit either of the
             following groups (results from any UK lab are acceptable):

               -  (MR4 group) all the available BCR-ABL1 molecular results over the preceding 12
                  months are in MR4 (MR4 is defined as a BCR-ABL1/ABL1 ratio of zero, (reported to
                  International Standards (IS) where possible; with at least 10,000 ABL1 control
                  transcripts).

               -  (MMR group) some or all BCR-ABL1 molecular results are in major molecular
                  response (MMR), defined here as a BCR-ABL1/ABL1 ratio of 0.1% or less, (reported
                  to International Standard (IS) where possible), but not zero, with at least
                  10,000 ABL1 control transcripts. If the results over the preceding 12 months are
                  a mix of MMR and undetectable BCR-ABL1, then the patient is eligible for the MMR
                  but not the MR4 group.

        Exclusion Criteria:

          1. Age under 18

          2. Life expectancy predicted to be less than 37 months because of intercurrent illness

          3. Presence of serious concomitant illness (e.g. heart, renal, respiratory or active
             malignant disease) that might preclude completion of the trial

          4. CML in accelerated phase or blast crisis at any time

          5. Any molecular result during the preceding 12 months that is not in either MMR or MR4.

          6. Patients who switched previous licensed TKI treatment (imatinib, nilotinib or
             dasatinib) twice or more because of intolerance

          7. Treatment with higher than standard TKI doses ('standard' is defined as imatinib 400mg
             daily, nilotinib 400mg twice daily or dasatinib 100mg daily). However, an exception is
             made for patients who at original diagnosis commenced on either 800mg of imatinib on
             the SPIRIT1 study, or 140mg (or 70mg b.d) of dasatinib in the Bristol-Myers Squibb 034
             study. In each case these latter patients ARE eligible provided they fulfil other
             molecular criteria, since they do not demonstrate resistant disease.

          8. Patients who switched previous licensed TKI treatment (imatinib, nilotinib or
             dasatinib) because of resistance. Patients treated with lower (but at least 50%) than
             the standard TKI doses (as defined in previous criterion) for tolerance reasons may be
             included, but will de-escalate to the same doses as for standard dose patients and
             will be analysed separately, as they could be seen as undertreated.

          9. Previous treatment with ponatinib or bosutinib. Patients who received interferon prior
             to commencing TKI (even if resistant to their interferon) are eligible, provided their
             response to TKI fits the entry criteria.

         10. Pregnant or lactating women

         11. Women of childbearing potential (including women whose last menstrual period was less
             than one year prior to screening) who are unable or unwilling to use adequate
             contraception from study start to one year after the last dose of protocol therapy.
             Adequate contraception is defined as hormonal birth control, intrauterine device,
             double barrier method or total abstinence.