Peripheral blood cells from patient X09, a 6-year old boy newly diagnosed with an immature CD4/CD8 double negative T-ALL, underwent whole-genome and RNA sequencing, revealing a translocation between chromosomes 5 and 11 and co-occurring mutations in NRAS, NOTCH1, Krüppel-like factor 9 (KLF9), Cyclin-dependent kinase inhibitor 2A (CDKN2A), Sidekick cell adhesion molecule 1 (SDK1), T-cell receptor gamma (TRG), and transducin (beta)-like 1 X-linked receptor 1 (TBL1XR1) (Fig. 1a). Here, KLF9 is linked to acute lymphoblastic leukemia.