NOTCH1 PEST domain mutations are frequently found in T-cell acute lymphoblastic leukemia (T-ALL). PEST domain mutations interfere with ubiquitination-mediated NOTCH1 downregulation and result in prolonged half-life of the intracellular NOTCH1 fragment, NICD1, and increased NICD1 transcriptional activity (Weng et al. 2004, Thompson et al. 2007, O'Neil et al. 2007).