MYC and acute lymphoblastic leukemia: Here, we show that IL7R gain-of-function alone is sufficient to trigger T-cell leukemogenesis in zebrafish, a process that involves increased IL-7R-mediated signaling as well as transcriptional activation of MYC. In agreement, mutant IL-7Rα also acts as a collaborating oncogene that synergizes with Myc to drive early-onset T-ALL.