The MEK/ERK pathway is activated downstream of driver oncogenes such as BCR-ABL1 and NRAS,6 and accordingly, we observed different levels of MEK/ERK pathway activation not only in six B-ALL cell lines expressing BCR-ABL1+ or NRASG12D but also in four cell lines driven by other oncoproteins, including ETV6-PDGFRB, ETV6-RUNX1, and MLL-AF4, except RS4;11 cells, which demonstrated no MEK/ERK pathway activity (Figures 1a and b; Supplementary Figures S1a and b; Supplementary Table S1). Here, AFF1 is linked to acute lymphoblastic leukemia.