ABT-199, also known as GDC-0199, is primarily used in the treatment of certain types of cancers, particularly chronic lymphocytic leukemia (CLL) and small lymphocytic lymphoma (SLL). It functions as a selective BCL-2 inhibitor, which helps in promoting apoptosis, or programmed cell death, in cancer cells. This mechanism is especially effective in cancers where BCL-2 proteins are overexpressed, as these proteins typically prevent apoptosis and allow cancer cells to survive. ABT-199 is often employed when other treatments have failed or are not suitable, offering a targeted approach to disrupt the survival of malignant cells. Its use has shown significant promise in improving patient outcomes, particularly in relapsed or refractory cases, making it a valuable option in oncology therapeutics.