Background And Aims: SintraRev is the first phase III, multicenter clinical trial evaluating lenalidomide(Len) in del(5q)-MDS anemic patients, prior to develop transfusion dependency. After having confirmed the clinical benefit of early treatment, we evaluated the safety of this early approach regarding clonal evolution. Methods: Sixty-one patients were randomized (2:1) in a double-blind design to Len (5 mg/day) vs placebo (Pcb) for 2 years of treatment and 2 years of follow-up. NGS was performed in available samples at baseline, week 12 and end of treatment. A capture-based NGS panel targeting 65 myeloid genes was used. Libraries were prepared following manufacturer’s instructions (Twist BioScience) and run on Illumina sequencers. Results: NGS data was available in forty-seven patients (29Len, 18Pcb) with median follow-up of 60.6 (32.3-73.9) months. Baseline clinical and molecular features were similar in patients from both groups (Table 1). The most frequently mutated genes were SF3B1 (25.5%) and TP53 (21.3%). At week 12, a significant reduction in VAF’s mutations was observed in the Len group (from 14% to 7%, p = 0.01) that persisted until Len discontinuation. Conversely, mutations in Pcb patients showed a significant increase of the median VAFat the end of treatment phase (29.5%) (Figure 1). In the Len group, all SF3B1mut patients achieved response to treatment. In addition, TP53mut did not show association to AML progression neither earlier transformin time than TP53wt. Results will be expanded in the meeting. Conclusions: Early administration of Len in del(5q)-MDS patients without TD did not promote clonal evolution of preexisting clones. As well, TP53 mutations did not accelerated AML progression in patients treated with Len.