NOTCH1 and B-cell chronic lymphocytic leukemia: These observations are consistent with past work showing that exon 34 mutations producing C-terminal PEST domain deletions have no effect on receptor activation per se [3,28]; thus, they are only selected for in contexts in which there is already ongoing NOTCH1 activation, which may be caused by mutations in exons 25-28 (apparently absent in CLL) or by ligand-mediated NOTCH1 activation.