The primary objective of the study is to describe the stability of molecular response with NIL as frontline therapy in newly diagnosed, unselected, CP CML patients, in an independent, investigator-initiated observational study. Imatinib mesylate (IM), a protein tyrosine kinase inhibitor (TKI) targeting BCR-ABL, has become in the last decade the standard of care for Chronic Myeloid Leukaemia (CML) in chronic phase (CP)1,2. Nilotinib (NIL) is a second generation TKI, effective in IM-resistant and IM-intolerant patients, which demonstrated superior efficacy to IM in early CP BCR-ABL+ CML patients3. Currently, the most important target of the treatment of CML with TKIs is the major molecular response (MMR), defined as a ≥ 3 log reduction in BCR-ABL/ABL transcript level, marker of better long-term outcome. With imatinib therapy, achieving a MMR correlates with an improved probability of a durable cytogenetic remission30. Results from IRIS suggest that a MMR after 12 months of imatinib therapy may be a marker of stable response. Further on, the IRIS study showed that patients with a MMR after 12 months of therapy had a significantly better probability of disease-free survival compared with those in complete cytogenetic remission, but not in MMR31. Moreover, obtaining an undetectable BCR-ABL transcript level is extremely relevant in order to consider TKIs discontinuation. This condition is known as "Complete Molecular Response" (CMR) and is further defined according to the sensitivity achieved (for the definition see the "Criteria of evaluation" section). As far as treatment discontinuation, two experiences have been published so far, aimed at evaluating the persistence of the CMR after imatinib discontinuation. The first was a pilot study32 where 12 patients were included. These 12 patients discontinued imatinib after at least 2 years of CMR (median duration of negativity, 32 months). Six patients displayed a molecular relapse with a detectable BCR-ABL transcript at 1, 1, 2, 3, 4, and 5 months. Imatinib was then reintroduced and led to a novel molecular response. Six other patients (50%) still have an undetectable level of BCR-ABL transcript after a median follow-up of 18 months (range, 9-24 months). The results of this pilot trial have been confirmed and extended in a second trial, the STIM trial33: 100 patients were enrolled, median follow-up 17 months, 69 patients with at least 12 months follow-up: 42 (61%) of these 69 patients relapsed (40 before 6 months, one patient at month 7, and one at month 19). At 12 months, the probability of persistent CMR for these 69 patients was 41% (95% CI 29-52). All patients who relapsed responded to reintroduction of imatinib. An increase of the CMR rate could possibly translate in a higher proportion of patients candidate to stopping anti-CML treatment, with higher probability of remaining disease-free in the long term. The advantages of this possible future scenario could be: first, the possibility of treatment discontinuation at least in patients with chronic clinical adverse events; second, a potential reduction of the costs of TKI treatment (after the introduction of TKI, the costs of CML treatment is increasing year by year, with the increasing prevalence of CML patients). Standardized molecular monitoring has become widely available in Europe through the efforts of EUTOS cooperation19 and now allows for the generation of comparable data on the residual disease using recalculation on the international scale despite these data being analyzed in many different laboratories. These advances in the standardization of molecular responses and the improvement of targeted therapy have allowed for comparable response assessment across Italian Centres and early treatment optimization of patients.

In summary, 1) monitoring of molecular response and of deep and sustained molecular response, provides a straightforward opportunity to assess patients' response and possible prognosis in the use of targeted therapy. 2) Most data on second generation TKIs are from company-sponsored studies generally implemented in selected referral centres. 3) The detailed description of the kinetic of the molecular response and, particularly, the rate of stable MR4, potentially related in turn with a subsequent treatment discontinuation, within the frame of a long-term post-marketing surveillance observational trial offered to all eligible patients followed at a nation-wide, independent multicentre group is the core distinctive feature of this observational trial.