AFF1 and leukemia: Moreover, even though only 50% of the analysed paired samples presented a mutation also at relapse, a percentage that still raises some questions about the overall role of RAS mutations in MLL-AF4+ leukaemia, the mutated cells with a total VAF >10% seemed to show a certain dependency on the presence of an active RAS signalling, which may suggest that RAS mutations can sustain proliferative pathways already active in the leukaemic cells.