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The prognostic significance of BCR-ABL1 transcripts in chronic myeloid leukemia (CML) is controversial. A recent report demonstrated that patients with e13a2 transcripts have inferior outcomes with imatinib 400 mg and those patients with e14a2 or that express both transcripts have more chance of an optimal response and longer event-free and transformation-free survival, while others do not confirm this data.The aim of this study was to evaluate the impact of BCR-ABL transcript type in CML patients outcome.Patients and methods: all consecutive CML patients in chronic phase treated with imatinib 400 mg/day from February 2004 to January 2016 were enrolled. Patients' responses were monitored with cytogenetic analysis at 3, 6 and 12 months, then every six months until a complete cytogenetic response (CCR). Real-time polymerase chain reaction was assessed at baseline, then every 3 months for the first year until reaching a stable major molecular response, then every 3-6 months. Demographic and baseline disease characteristics were collected at diagnosis. The type of BCR-ABL1 transcript was evaluated by multiplex RT-PCR from cDNA synthesized from total leukocytes RNA at diagnosis. We included patients with BCR-ABL transcripts e13a2, e14a2, and with coexpression of e13a2 and e14a2. Statistical analysis: Event-free survival (EFS) was measured from the start of treatment until loss of complete hematologic response, loss of major cytogenetic response, progression to accelerated (AP) or blast phase (BC) or death from any cause at any time while on initial therapy. Overall survival (OS) was measured from the start of imatinib until to the date of death from any cause at any time or last follow-up. Transformation-free survival (TFS) was measured from the start of imatinib to transformation to AP or BC or deaths while on therapy. The differences between variables were analyzed by the χ2 and the Kruskal-Wallis tests for categorical and continuous variables. Survival probabilities were calculated using the Kaplan-Meier method and compared by the log-rank test. The Cox regression estimated the hazard ratio values. All analysis considering p-value < 0.05 and using SPSS 21.0 software. Results: A total of 190 patients were treated with imatinib 400 mg/day. Median age was 48 years (18-87) and Sokal risk was high in 47/151 patients (31%), intermediate in 55/151 (36%) and low in 49/151 (33%). Twenty patients were excluded from the analysis: 14 patients due to Interferon treatment before Imatinib; two patients that started imatinib after six months from diagnosis; two patients with e1a2 transcripts and two patients with no RT-PCR test available at diagnosis. The remaining 170 patients presented typical BCR-ABL1 transcripts: e13a2 (n=56; 33%), e14a2 (n=94; 55%) and both transcripts (n=20; 12%). A total of 44 (26%) patients discontinued imatinib and 24 (14%) switched to second-line tyrosine kinase inhibitor. No differences were observed in sex, age, leukocytes, hemoglobin and platelets count at diagnosis, Sokal or EUTOS score according to transcript type. The proportion of patients with e13a2, e14a2 and both achieving complete cytogenetic response at 6 months was 19/44 (43%); 42/60 (70%) and 9/14 (64%) (P=0.02); and at 12 months was 28/45 (62%); 47/60 (78%) and 11/14 (78%) (P=0.16). However, the proportion of patients with major or lower molecular responses at 18 months was 13/24 (54%), 25/36 (69%) and 6/9 (66%), which was not significantly different. There were no statistical difference in EFS, PFS, and OS among the e13a2, e14a2 and e14a2+e13a2 groups. However, there was a superior 10-year overall survival in patients with transcripts e13a2 compared to e14a2 (alone or coexpressed with e13a2) (93% vs. 73%, P=0.03), although the 5-year overall survival was 96% vs. 88%, respectively (P=NS). In the multivariate analysis, high/intermediate Sokal score and e14a3/e14a3+e14a2 were independent factors for poor OS (P=0.023 and 0.041, respectively).Conclusion: The type of BCR-ABL transcripts did not affect molecular responses. Although patients with e14a2 transcripts presented higher rates of CCR at 6 months, compared to e13a2 or both transcripts, at long term, there was a superior overall survival among patients with transcripts e13a2 compared to e14a2 (alone or coexpressed with e13a2). The biological mechanism responsible for that difference is not known and should be investigated in larger trials. Disclosures Pagnano: Novartis: Consultancy, Honoraria; Bristol-Meyers Squibb: Consultancy, Honoraria.