This study investigates the role of PTEN in the resistance to NOTCH1 inhibition in T-cell leukemia (T-ALL). NOTCH1 signaling is a key regulator of cell growth and metabolism during T-cell development and transformation, and gain-of-function mutations in NOTCH1 are common in T-ALL. The authors found that NOTCH1 regulates PTEN expression and the activity of the PI3K-AKT signaling pathway in normal and leukemic T cells. Mutational loss of PTEN was associated with resistance to NOTCH1 inhibition in human T-ALL. In a Drosophila model of Notch-induced tumorigenesis, NOTCH1 signaling and the PI3K-AKT pathway synergized. Aberrant activation of the PI3K-AKT pathway induced resistance to NOTCH1 inhibition in T-ALL cells. The study also showed that HES1 and MYC regulate PTEN expression downstream of NOTCH1. In Drosophila, activation of Akt1 restored growth and cell proliferation upon Notch inhibition, and aberrant Akt1 activity synergized with Notch hyperactivation to promote tumor development. The findings suggest that the regulatory circuit linking NOTCH1 signaling with PTEN expression and PI3K-AKT activity may contribute to NOTCH1-induced transformation and could mediate the cellular response to NOTCH1 inhibitors in T-ALL.This study investigates the role of PTEN in the resistance to NOTCH1 inhibition in T-cell leukemia (T-ALL). NOTCH1 signaling is a key regulator of cell growth and metabolism during T-cell development and transformation, and gain-of-function mutations in NOTCH1 are common in T-ALL. The authors found that NOTCH1 regulates PTEN expression and the activity of the PI3K-AKT signaling pathway in normal and leukemic T cells. Mutational loss of PTEN was associated with resistance to NOTCH1 inhibition in human T-ALL. In a Drosophila model of Notch-induced tumorigenesis, NOTCH1 signaling and the PI3K-AKT pathway synergized. Aberrant activation of the PI3K-AKT pathway induced resistance to NOTCH1 inhibition in T-ALL cells. The study also showed that HES1 and MYC regulate PTEN expression downstream of NOTCH1. In Drosophila, activation of Akt1 restored growth and cell proliferation upon Notch inhibition, and aberrant Akt1 activity synergized with Notch hyperactivation to promote tumor development. The findings suggest that the regulatory circuit linking NOTCH1 signaling with PTEN expression and PI3K-AKT activity may contribute to NOTCH1-induced transformation and could mediate the cellular response to NOTCH1 inhibitors in T-ALL.