To obtain stable Nalm6+RUNX1 or Nalm6+ETV6-RUNX1 cell lines, Halotag-RUNX1 human ORF from pFN21A (#FHC01784, Kazusa collection, Promega), or Halotag-ETV6-RUNX1 (ETV6-RUNX1 ORF subcloned from plasmid kindly provided by G. Nucifora ) were cloned into a pLenti CMV-Puro-DEST by Gateway technology. Lenti CMV Puro DEST (w118–1) was a gift from Eric Campeau (Addgene plasmid # 17452) . To produce lentivirus, HEK293T cells were co-transfected with pLenti-CMV-Puro-DEST bearing Halotag-RUNX1 or Halotag-ETV6-RUNX1, pSPAX2 and pCMV-VSV-G for packaging using Lipofectamine 3000 Transfection Reagent (Thermo Fisher Scientific). The plasmid psPAX2 was a gift from Didier Trono (Addgene plasmid # 12260) and pCMV-VSV-G was a gift from Bob Weinberg (Addgene plasmid # 8454) . After 48 h, supernatant was harvested, filtered and added to Nalm6 cells with 4 μg/mL polybrene. All the transduced cells were selected in medium containing 0.25 μg/mL puromycin (Invitrogen) as previously established .