Used primarily in the treatment of acute lymphoblastic leukemia (ALL), especially in maintenance therapy to prevent relapse. It is also prescribed for other forms of leukemia and autoimmune disorders such as Crohn's disease and ulcerative colitis when other treatments fail. Acts as an immunosuppressant by inhibiting purine synthesis, thereby reducing the proliferation of rapidly dividing immune cells and cancer cells. Often used in combination with other chemotherapy agents or following induction therapy to sustain remission. Requires careful monitoring due to potential liver toxicity and myelosuppression.