Loss of function mutations found in FBXW7 in T-cell acute lymphoblastic leukemia are predominantly dominant negative missense mutations that target one of the three highly conserved arginine residues in the WD repeats of FBXW7 (Thompson et al. 2007, O'Neil et al. 2007). These three arginine residues are part of the FBXW7 substrate binding pocket and each one of them contacts the phosphorylated threonine residue in the conserved substrate phosphodegron region (Orlicky et al. 2003). Specifically, FBXW7 interacts with the PEST domain of NOTCH1 upon phosphorylation of the PEST domain by CDK8 (Fryer et al. 2004). FBXW7 mutants are therefore unable to bind and promote ubiquitination of the NOTCH1 intracellular domain (NICD1), leading to prolonged NICD1 transcriptional activity (Thompson et al. 2007, O'Neil et al. 2007).