a, Schematic representation of human NOTCH1, showing the main domains and locations of the three different somatic mutations identified in CLL. NEC, NOTCH1 extracellular subunit; NTM, NOTCH1 transmembrane subunit; ICN, intracellular domain of NOTCH1; LNR, Lin-12 NOTCH repeats; RAM, RAM domain; ANK, ankyrin repeat domain; PEST, PEST domain. b, Electropherogram showing the heterozygous CT deletion recurrently identified in CLL. c, Western blot showing NOTCH1 protein levels in CLL cases with or without the NOTCH1 p.P2515Rfs*4 mutation, and in Jurkat cells as a control. The arrow indicates the band corresponding to the NTM; the large arrowhead indicates the smaller band corresponding to the mutant form. d, Heat map showing the 23 genes of the NOTCH1 pathway that are differentially expressed in NOTCH1-mutated versus non-mutated CLL. e, Distribution of disease stage (Binet), ZAP-70 expression status, CD38 expression status and IGHV mutational status (UM, unmutated IGHV) in patients with or without mutations in NOTCH1 (*, P < 0.02; **, P < 0.01). f, Actuarial probability of overall survival of CLL patients with mutated or unmutated NOTCH1 (*, P = 0.03).