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Abstract Background Activated mutations in NOTCH1 are drivers of T-cell type acute lymphoblastic leukemia/lymphoma. The γ-secretase inhibitor (GSI), which suppresses the function of NOTCH1, is expected to be a molecular-targeted agent. NOTCH1 is also expressed in other malignant neoplasms. We aimed to determine the function of NOTCH1 expression and the effects of GSI on adult T-cell leukemia/lymphoma (ATL) caused by long-term human T-cell leukemia virus type I (HTLV-1) infection. Methods We analyzed the expression of NOTCH1 in six ATL- and HTLV-1-infected cell lines and investigated the influence of activated NOTCH1 (i.e., the cleaved form of NOTCH1) together with GSI on cell proliferation. Results Activated NOTCH1 found in ATL- and HTLV-1-infected cell lines was undetectable after incubation with GSI, regardless of Tax expression (HTLV-1-coded protein). Whole-exome sequencing revealed that activated NOTCH1 mutations were undetectable in six ATL- and HTLV-1-infected cell lines, regardless of abundant NOTCH1 expression. Moreover, GSI did not suppress the growth of ATL cell lines. Conclusions These findings suggested that NOTCH1 protein is constitutively activated but is likely a passenger during NOTCH1-mutation-negative ATL cell proliferation.