Notch signaling is of crucial importance in normal T-cell development and Notch 
1 is frequently mutated in T-cell acute lymphoblastic leukemias (T-ALL), leading 
to aberrantly high Notch signaling. In this report, we determine whether T-ALL 
mutations occur not only in Notch1 but also in the F-box protein hCdc4 (Sel-10, 
Ago, or Fbxw7), a negative regulator of Notch1. We show that the hCDC4 gene is 
mutated in leukemic cells from more than 30% of patients with pediatric T-ALL 
and derived cell lines. Most hCDC4 mutations found were missense substitutions 
at critical arginine residues (Arg(465), Arg(479), and Arg(505)) localized in 
the substrate-binding region of hCdc4. Cells inactivated for hCdc4 and T-ALL 
cells containing hCDC4 mutations exhibited an increased Notch1 protein 
half-life, consistent with the proposed role of hCdc4 in ubiquitin-dependent 
proteolysis of Notch1. Furthermore, restoration of wild-type but not mutant 
hCdc4 in HCT 116 hCDC4-negative cells led to an increased Notch1 ubiquitylation 
and decreased Notch1 signaling. These results show that hCdc4 mutations 
interfere with normal Notch1 regulation in vivo. Finally, we found that 
mutations in hCDC4 and NOTCH1 can occur in the same cancers and that patients 
carrying hCDC4 and/or NOTCH1 mutations have a favorable overall survival. 
Collectively, these data show that mutation of hCDC4 is a frequent event in 
T-ALL and suggest that hCDC4 mutations and gain-of-function mutations in NOTCH1 
might synergize in contributing to the development of pediatric T-ALL 
leukemogenesis.