Activating NOTCH1 mutations occur in over 50% of T-ALL and T-LBL cases. The majority of these occur in the heterodimerization or PEST domain (black arrows) and are associated with favorable outcomes in T-ALL and T-LBL. The translocation of NOTCH1 gene to TRB locus t(7;9) results in aberrant NOTCH activation. te Vrugt et al show this fusion is identified in T-LBL, but not T-ALL, and its presence correlates with higher relapse risk. chrm, chromosome; TCR, T-cell receptor.