This study will evaluate the proportion of subjects with chronic myeloid leukemia chronic phase that sustain major molecular response after imatinib discontinuation. To be eligible for this protocol, the subject must have received imatinib as first line regiment for at least 3 years with sustained molecular response of 4log (RM4log) or higher for one year.

Patients will be followed for 2 years, with molecular monitoring, every month in the first year and then every 2 months. Imatinib will be restarted if the major molecular response (RM3log) is lost during the study or if the subject withdraw the informed consenting.

The primary objective is to measure the number of subject that sustain the major molecular response (RM3log or BCR-ABL level below 0.1%) after 2 years of follow-up.

BCR: breakpoint cluster region

ABL: abelson murine leukemia

RM3log: major molecular response, BCR-ABL level below 0.1% (IS)

Inclusion Criteria:

• Chronic myeloid leukemia chronic-phase, defined by the World Health Organization criteria
• Treatment with imatinib for at least 36 months
• BCR-ABL levels below 0,01% (IS) or MR4log in the last 12 months

Exclusion Criteria:

• Previous allogeneic stem cell transplantation
• Previous treatment with dasatinib, nilotinib, bosutinib or ponatinib
• Imatinib dose escalation at any time, due to loss or inadequate response
• BCR-ABL mutation

IS: International Scale

MR4log: molecular response of 4log or <0,1% (IS)