ETV6-NCOA2 induces a T/myeloid hematopoietic malignancy accompanied by spontaneous Notch1 mutations in C57BL/6 mice. C57BL/6 mice BM progenitors treated with fluorouracil were transduced with MSCV-MIGR1-IRES-GFP retroviruses expressing EN2 (n = 18), KN2 (n = 3), or empty vector (EV) (n = 16). (A) Kaplan-Meier tumor-free survival analysis of EN2 primary, secondary (n = 4), and tertiary (n = 2) mice compared with KN2 and empty vector (log-rank test EN2 vs empty vector P = .0016). (B) Examples of EN2 tumors. (C) Flow cytometry analysis of KN2- and EN2-transduced leukemic cells. Top: flow examples of CD45, GFP, CD11b/Thy1.2, CD4/CD8, and CD44/CD25. Bottom: summary of Thy1.2 and CD11b flow cytometry results (Mann-Whitney U test **P = .0095). (D) Two examples of Notch1 mutations in the PEST domain detected in EN2 leukemia/lymphoma in C57BL/6 mice. APC, allophycocyanin; PE, phycoerythrin.