Prevalence of activating mutations in NOTCH1 in T-ALL.NOTCH1 HDand ΔPESTmutations account for the majority of activating mutations in NOTCH1 in T-ALL. An additional 1% of patients harbor translocations involving NOTCH1 and the TCR loci. NOTCH1 JME alleles are found in 3% of all T-ALLs. In total about 15% of T-ALL cases harbor deletions or mutations in FBXW7, which impair the degradation of activated NOTCH1 in the nucleus and are functionally related to NOTCH1 ΔPEST mutant alleles.