NOTCH1 and acute lymphoblastic leukemia: Compared to wild-type thymus-derived T-ALL, Notch1DECRREE thymus-derived T-ALL development was markedly delayed with overall significantly decreased disease penetrance (Fig. 2B), indicating that RSS-RAG-mediated genomic 5′ deletions are a dominant mechanism of Notch1 activation and resultant leukemogenesis in mice.