ONO-4059 (GS-4059) hydrochloride is primarily used in the field of medical research, particularly in the study and treatment of cancer. It functions as a potent and selective inhibitor of Bruton's tyrosine kinase (BTK), an enzyme crucial for the survival and proliferation of certain cancer cells, especially in B-cell malignancies like chronic lymphocytic leukemia (CLL) and non-Hodgkin lymphoma (NHL). By blocking BTK, it disrupts signaling pathways that promote tumor growth, making it a promising candidate for targeted cancer therapy. Additionally, it is being investigated for its potential in treating autoimmune diseases due to its role in modulating immune cell activity. Its application extends to preclinical and clinical studies aimed at understanding its efficacy, safety, and therapeutic potential in various disease models.